Your next step should be to check for spyware, at least to eliminate the possibility.
Download HijackThis from http://209.133.47.200/~merijn/files/HijackThis.exe & unzip it into it's own, permanent folder, not a temporary one. Start HJT & press the scan button. When the scan is finished the scan button will change to save. Save the log to a text file & paste it into the body of your post. DO NOT FIX ANYTHING YET.
I hope this helps

Thanks
Logfile of HijackThis v1.97.7
Scan saved at 9:16:39 AM, on 4/13/2004
Platform: Windows XP SP1 (WinNT 5.01.2600)
MSIE: Internet Explorer v6.00 SP1 (6.00.2800.1106)
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\system32\spoolsv.exe
C:\Program Files\Trend Micro\PC-cillin 2003\Tmntsrv.exe
C:\Program Files\Intel\Intel(R) Active Monitor\imonnt.exe
C:\Program Files\Trend Micro\PC-cillin 2003\tmproxy.exe
C:\Program Files\Trend Micro\PC-cillin 2003\PccPfw.exe
C:\WINDOWS\Explorer.EXE
C:\Program Files\Intel\Intel(R) Active Monitor\imontray.exe
C:\Program Files\Trend Micro\PC-cillin 2003\pccguide.exe
C:\Program Files\Trend Micro\PC-cillin 2003\PCCClient.exe
C:\Program Files\Messenger Plus! 2\MsgPlus.exe
C:\PROGRA~1\iolo\SYSTEM~1\PopupStopper.exe
C:\Program Files\TuneUp Utilities 2004\MemOptimizer.exe
C:\Program Files\Internet Explorer\iexplore.exe
C:\Program Files\MSN Messenger\msnmsgr.exe
C:\Program Files\DC++\DCPlusPlus.exe
C:\Program Files\Messenger\msmsgs.exe
C:\HighJackThis\HijackThis.exe
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Start Page =
http://www.yahoo.com/
R0 - HKLM\Software\Microsoft\Internet Explorer\Search,SearchAssistant =
http://mysearchnow.com/searchbar.html
R0 - HKCU\Software\Microsoft\Internet Explorer\Main,Local Page =
O2 - BHO: (no name) - {A5366673-E8CA-11D3-9CD9-0090271D075B} - (no file)
O3 - Toolbar: &Radio - {8E718888-423F-11D2-876E-00A0C9082467} - C:\WINDOWS\System32\msdxm.ocx
O3 - Toolbar: FlashGet Bar - {E0E899AB-F487-11D5-8D29-0050BA6940E3} - C:\PROGRA~1\FLASHGET\fgiebar.dll
O4 - HKLM\..\Run: [IMONTRAY] C:\Program Files\Intel\Intel(R) Active Monitor\imontray.exe
O4 - HKLM\..\Run: [pccguide.exe] "C:\Program Files\Trend Micro\PC-cillin 2003\pccguide.exe"
O4 - HKLM\..\Run: [PCCClient.exe] "C:\Program Files\Trend Micro\PC-cillin 2003\PCCClient.exe"
O4 - HKLM\..\Run: [MessengerPlus2] "C:\Program Files\Messenger Plus! 2\MsgPlus.exe"
O4 - HKCU\..\Run: [System Mechanic Popup Stopper] "C:\PROGRA~1\iolo\SYSTEM~1\PopupStopper.exe"
O4 - HKCU\..\Run: [TuneUp MemOptimizer] "C:\Program Files\TuneUp Utilities 2004\MemOptimizer.exe" autostart
O4 - HKCU\..\Run: [Window Washer] C:\Program Files\Webroot\Washer\wwDisp.exe
O4 - HKLM\..\RunOnce: [Index Washer] C:\Program Files\Webroot\Washer\WashIdx.exe "Christine"
O4 - HKCU\..\RunOnce: [Index Washer] C:\Program Files\Webroot\Washer\WashIdx.exe "Christine"
O8 - Extra context menu item: &NeoTrace It! - C:\PROGRA~1\NEOTRA~1\NTXcontext.htm
O8 - Extra context menu item: Download All by FlashGet - C:\PROGRA~1\FLASHGET\jc_all.htm
O8 - Extra context menu item: Download using FlashGet - C:\PROGRA~1\FLASHGET\jc_link.htm
O9 - Extra button: ICQ Pro (HKLM)
O9 - Extra 'Tools' menuitem: ICQ (HKLM)
O9 - Extra button: FlashGet (HKLM)
O9 - Extra 'Tools' menuitem: &FlashGet (HKLM)
O9 - Extra button: NeoTrace It! (HKCU)
O16 - DPF: {0E5F0222-96B9-11D3-8997-00104BD12D94} (PCPitstop Utility) -
http://www.pcpitstop.com/pcpitstop/PCPitStop.CAB
O16 - DPF: {166B1BCA-3F9C-11CF-8075-444553540000} (Shockwave ActiveX Control) -
http://download.macromedia.com/pub/s...irector/sw.cab
O16 - DPF: {3AF4DACE-36ED-42EF-9DFC-ADC34DA30CFF} (PatchInstaller.Installer) -
file://D:\content\include\XPPatchInstaller.CAB
O16 - DPF: {9F1C11AA-197B-4942-BA54-47A8489BB47F} (Update Class) -
http://v4.windowsupdate.microsoft.co...8077.313275463
O16 - DPF: {D27CDB6E-AE6D-11CF-96B8-444553540000} (Shockwave Flash Object) -
http://download.macromedia.com/pub/s...sh/swflash.cab