I had this exact problem, and it persisted and got worse each time i went to the computer. It was like a downward spiral. I eventually Formatted and Installed a fresh copy of XP Pro. YOu should back up your files and do a factory restore in my opinion. All the executables I backed up apparently contained a virus when I tried to use them on my fresh XP install. So I couldn't access them without risking infection again. It's most likely a virus.