Gracias Heavyman, hice o intenté hacer todo lo que me sugeriste.
Te cuento: Descargué e instalé todos los programas. Con respecto al AVG, lo instalé sin problemas pero, yo ya tenía instalado el Avast. Así que quedaron los dos. El AVG anduvo bien al principio pero, ahora ya no es posible ejecutarlo desde ninguno de los íconos del escritorio. Los clickeo pero, no ejecutan nada. Realicé todos los pasos pero, tuve algunos inconvenientes. Me fue imposible encontrar el archivo mdmex5.exe por lo que no pude eliminarlo. Acá va el log de HJT que acabo de correr.

Logfile of HijackThis v1.99.1
Scan saved at 10:06:05 a.m., on 11/06/2007
Platform: Windows XP SP2 (WinNT 5.01.2600)
MSIE: Internet Explorer v7.00 (7.00.6000.16441)

Running processes:
C:\WINDOWS\System32\smss.exe
C:\WINDOWS\system32\winlogon.exe
C:\WINDOWS\system32\services.exe
C:\WINDOWS\system32\lsass.exe
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\System32\svchost.exe
C:\WINDOWS\Explorer.EXE
C:\Program Files\Alwil Software\Avast4\aswUpdSv.exe
C:\Program Files\Alwil Software\Avast4\ashServ.exe
C:\WINDOWS\system32\spoolsv.exe
C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
C:\Program Files\ATI Technologies\ATI Control Panel\atiptaxx.exe
C:\WINDOWS\system32\carpserv.exe
C:\Program Files\Java\jre1.6.0_01\bin\jusched.exe
C:\Program Files\ABERA\IrDA\IrServer.exe
C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\avgas.exe
C:\WINDOWS\system32\ctfmon.exe
C:\Program Files\Google\GoogleToolbarNotifier\GoogleToolbarNo tifier.exe
C:\Program Files\Common Files\DataViz\DvzIncMsgr.exe
C:\Program Files\palmOne\Hotsync.exe
C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\guard.exe
C:\WINDOWS\system32\cisvc.exe
C:\WINDOWS\system32\E_S00RP1.EXE
C:\WINDOWS\system32\inetsrv\inetinfo.exe
C:\Program Files\Linksys\Wireless-G Notebook Adapter\NICServ.exe
C:\WINDOWS\system32\tcpsvcs.exe
C:\WINDOWS\System32\snmp.exe
C:\WINDOWS\system32\svchost.exe
C:\WINDOWS\system32\mqsvc.exe
C:\WINDOWS\system32\mqtgsvc.exe
C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe
C:\Program Files\Alwil Software\Avast4\ashWebSv.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\WINDOWS\system32\cidaemon.exe
C:\WINDOWS\system32\cidaemon.exe
C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\avgas.exe
C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\avgas.exe
C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\avgas.exe
C:\Program Files\HJT\HijackThis[www.trucoswindows.net].exe

R3 - URLSearchHook: Yahoo! Toolbar - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O2 - BHO: Yahoo! Toolbar Helper - {02478D38-C3F9-4EFB-9B51-7695ECA05670} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O2 - BHO: Adobe PDF Reader Link Helper - {06849E9F-C8D7-4D59-B87D-784B7D6BE0B3} - C:\Program Files\Adobe\Acrobat 7.0\ActiveX\AcroIEHelper.dll
O2 - BHO: SSVHelper Class - {761497BB-D6F0-462C-B6EB-D4DAF1D92D43} - C:\Program Files\Java\jre1.6.0_01\bin\ssv.dll
O2 - BHO: Google Toolbar Helper - {AA58ED58-01DD-4d91-8333-CF10577473F7} - c:\program files\google\googletoolbar3.dll
O2 - BHO: Google Toolbar Notifier BHO - {AF69DE43-7D58-4638-B6FA-CE66B5AD205D} - C:\Program Files\Google\GoogleToolbarNotifier\2.0.301.7164\sw g.dll
O3 - Toolbar: 1-Click Answers - {7754C418-F62E-44aa-B169-E719E718BCFD} - C:\PROGRA~1\1-CLIC~1\IEToolbar\AnswersToolbarU.dll
O3 - Toolbar: &Google - {2318C2B1-4965-11d4-9B18-009027A5CD4F} - c:\program files\google\googletoolbar3.dll
O3 - Toolbar: Yahoo! Toolbar - {EF99BD32-C1FB-11D2-892F-0090271D4F88} - C:\Program Files\Yahoo!\Companion\Installs\cpn\yt.dll
O4 - HKLM\..\Run: [IMJPMIG8.1] "C:\WINDOWS\IME\imjp8_1\IMJPMIG.EXE" /Spoil /RemAdvDef /Migration32
O4 - HKLM\..\Run: [avast!] C:\PROGRA~1\ALWILS~1\Avast4\ashDisp.exe
O4 - HKLM\..\Run: [MsmqIntCert] regsvr32 /s mqrt.dll
O4 - HKLM\..\Run: [\\Bangho\EPSON Stylus CX4700 Series] C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\E_FATIA DL.EXE /P35 "\\bangho\EPSON Stylus CX4700 Series" /O6 "USB001" /M "Stylus CX4700"
O4 - HKLM\..\Run: [RoxioEngineUtility] "C:\Program Files\Common Files\Roxio Shared\System\EngUtil.exe"
O4 - HKLM\..\Run: [ATIPTA] C:\Program Files\ATI Technologies\ATI Control Panel\atiptaxx.exe
O4 - HKLM\..\Run: [CARPService] carpserv.exe
O4 - HKLM\..\Run: [modemspy] "C:\Program Files\Modem Spy\modemspy.dll" /tray
O4 - HKLM\..\Run: [Tweak UI] RUNDLL32.EXE TWEAKUI.CPL,TweakMeUp
O4 - HKLM\..\Run: [SunJavaUpdateSched] "C:\Program Files\Java\jre1.6.0_01\bin\jusched.exe"
O4 - HKLM\..\Run: [KernelFaultCheck] %systemroot%\system32\dumprep 0 -k
O4 - HKLM\..\Run: [NeroFilterCheck] C:\WINDOWS\system32\NeroCheck.exe
O4 - HKLM\..\Run: [EPSON Stylus CX4700 Series] C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\E_FATIA DL.EXE /P26 "EPSON Stylus CX4700 Series" /O6 "USB003" /M "Stylus CX4700"
O4 - HKLM\..\Run: [ABERA_IR] C:\Program Files\ABERA\IrDA\IrServer.exe
O4 - HKLM\..\Run: [Auto EPSON Stylus CX4700 Series on CARLOS] C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\E_FATIA DL.EXE /P41 "Auto EPSON Stylus CX4700 Series on CARLOS" /O19 "\\CARLOS\Impresora2" /M "Stylus CX4700"
O4 - HKLM\..\Run: [!AVG Anti-Spyware] "C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\avgas.exe" /minimized
O4 - HKCU\..\Run: [\\BANGHO\EPSON Stylus CX4700 Series] C:\WINDOWS\System32\spool\DRIVERS\W32X86\3\E_FATIA DL.EXE /P35 "\\BANGHO\EPSON Stylus CX4700 Series" /M "Stylus CX4700" /EF "HKCU"
O4 - HKCU\..\Run: [ctfmon.exe] C:\WINDOWS\system32\ctfmon.exe
O4 - HKCU\..\Run: [swg] C:\Program Files\Google\GoogleToolbarNotifier\GoogleToolbarNo tifier.exe
O4 - HKCU\..\Run: [NBJ] "C:\Program Files\Ahead\Nero BackItUp\NBJ.exe"
O4 - Global Startup: DataViz Inc Messenger.lnk = C:\Program Files\Common Files\DataViz\DvzIncMsgr.exe
O4 - Global Startup: HOTSYNCSHORTCUTNAME.lnk = C:\Program Files\palmOne\Hotsync.exe
O8 - Extra context menu item: Answers... - file:C:\Program Files\1-Click Answers\Html\atiemenu.htm
O8 - Extra context menu item: E&xportar a Microsoft Excel - res://C:\PROGRA~1\MICROS~2\OFFICE11\EXCEL.EXE/3000
O9 - Extra button: (no name) - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre1.6.0_01\bin\ssv.dll
O9 - Extra 'Tools' menuitem: Sun Java Console - {08B0E5C0-4FCB-11CF-AAA5-00401C608501} - C:\Program Files\Java\jre1.6.0_01\bin\ssv.dll
O9 - Extra button: Referencia - {92780B25-18CC-41C8-B9BE-3C9C571A8263} - C:\PROGRA~1\MICROS~2\OFFICE11\REFIEBAR.DLL
O9 - Extra button: (no name) - {e2e2dd38-d088-4134-82b7-f2ba38496583} - %windir%\Network Diagnostic\xpnetdiag.exe (file missing)
O9 - Extra 'Tools' menuitem: @xpsp3res.dll,-20001 - {e2e2dd38-d088-4134-82b7-f2ba38496583} - %windir%\Network Diagnostic\xpnetdiag.exe (file missing)
O9 - Extra button: Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exe
O9 - Extra 'Tools' menuitem: Windows Messenger - {FB5F1910-F110-11d2-BB9E-00C04F795683} - C:\Program Files\Messenger\msmsgs.exe
O11 - Options group: [INTERNATIONAL] International*
O16 - DPF: {30528230-99f7-4bb4-88d8-fa1d4f56a2ab} (YInstStarter Class) - C:\Program Files\Yahoo!\Common\yinsthelper.dll
O16 - DPF: {4F1E5B1A-2A80-42CA-8532-2D05CB959537} (MSN Photo Upload Tool) - http://gfx2.mail.live.com/mail/w1/resources/MSNPUpld.cab
O16 - DPF: {8AD9C840-044E-11D1-B3E9-00805F499D93} (Java Runtime Environment 1.6.0) - http://javadl-esd.sun.com/update/1.6.0/jinstall-6u1-windows-i586-jc.cab
O17 - HKLM\System\CCS\Services\Tcpip\..\{5BEA2898-1E0A-4A5D-8711-ECB86C2F37BC}: NameServer = 200.51.211.7,200.51.212.7
O17 - HKLM\System\CCS\Services\Tcpip\..\{AFBCEE58-A73F-4149-97C4-E62B8EE5CEF3}: NameServer = 200.51.211.7,200.51.212.7
O20 - Winlogon Notify: WgaLogon - C:\WINDOWS\SYSTEM32\WgaLogon.dll
O21 - SSODL: WPDShServiceObj - {AAA288BA-9A4C-45B0-95D7-94D524869DB5} - C:\WINDOWS\system32\WPDShServiceObj.dll
O23 - Service: avast! iAVS4 Control Service (aswUpdSv) - ALWIL Software - C:\Program Files\Alwil Software\Avast4\aswUpdSv.exe
O23 - Service: avast! Antivirus - ALWIL Software - C:\Program Files\Alwil Software\Avast4\ashServ.exe
O23 - Service: avast! Mail Scanner - Unknown owner - C:\Program Files\Alwil Software\Avast4\ashMaiSv.exe" /service (file missing)
O23 - Service: avast! Web Scanner - Unknown owner - C:\Program Files\Alwil Software\Avast4\ashWebSv.exe" /service (file missing)
O23 - Service: AVG Anti-Spyware Guard - Anti-Malware Development a.s. - C:\Program Files\Grisoft\AVG Anti-Spyware 7.5\guard.exe
O23 - Service: EPSON V3 Service2(03) (EPSON_PM_RPCV2_01) - SEIKO EPSON CORPORATION - C:\WINDOWS\system32\E_S00RP1.EXE
O23 - Service: Google Updater Service (gusvc) - Google - C:\Program Files\Google\Common\Google Updater\GoogleUpdaterService.exe
O23 - Service: InstallDriver Table Manager (IDriverT) - Macrovision Corporation - C:\Program Files\Common Files\InstallShield\Driver\11\Intel 32\IDriverT.exe
O23 - Service: NICSer_WPC54G - Unknown owner - C:\Program Files\Linksys\Wireless-G Notebook Adapter\NICServ.exe
O23 - Service: MS Software Shadow Copy Provider SwPrvSSDPSRV (SwPrvSSDPSRV) - Unknown owner - C:\WINDOWS\system32\22314.exe (file missing)