Try to run System File Checker first.
Start>Run then type CMD.EXE hit OK. Type SFC /scannow in the command prompt then hit Enter. You'll need to insert your XP CD.

Next try to use CWShredder. Download CWShredder. Download it from http://www.bleepingcomputer.com/files/Merijn/cwshredder.zip
Unzip to your desktop. Run CWShredder.exe and let it fix what it'll find. Reboot the system when done.

If problem is still there and Ad-aware, Stinger, SSD and NAV can't help, download HijackThis.

HijackThis download locations:

Current version of HijackThis is 1.98.2

Where to put and how to use HijackThis:

It is important that you will run HijackThis.exe in its own folder so the backup files that HijackThis file will create will not be accidentally deleted.

Open 'My Computer', then double-click to open C:\ (or the drive letter that your Windows is installed)
In the menu bar, click File-->New-->Folder.
That will create a folder named New Folder, which you can rename to "HJT" or "HijackThis". Now you have C:\HJT\ or C:\HijackThis\ folder. Put/extract your HijackThis.exe there, and double click to run it.

Click 'Scan' button. Click 'Save log' button. Save the 'hijackthis.log' in your desktop. Copy and paste the content of 'hijackthis.log' and post it the recommended sites that offers HijackThis analysis and is listed in http://www.a-sap.org/