Well, that makes it almost certain it's a virus... if the antivirus and/or antimalware found nothing, try
online virus scanns... and disable System Restore before scanning so the bugs won't have where to hide or save copies of themselfs. Also reset the default permissions with Subinacl following the instructions on the Winhelponline.com article, and apply all the suggestions you can from the thread linked below.
Reset the Registry and the File Permissions in Windows XP
http://www.winhelponline.com/blog/reset-the-registry-and-the-file-permissions-in-windows-xp/
Apply what you can from the suggestions on the best answer here:
http://www.tomshardware.com/answers/id-1863469/slow.html