First place would be to check for anything malicous. Do a virus and spyware scan. You can get some free ones here: [google]AVG Free Edition[/google] and [google]Spybot Search and Destroy[/google]. Check removeable media and other drives inase something has been left out by the format.
If you're still having problems update windows or probably easier still get [google]Windows XP SP2[/google]. You should be able to get a free CD ordered from the MS website.