Had similar problems - virus caused mine.
If running Windows XP turn off system restore before running virus checker/adaware/spyware type programme. There are now nasties that reside on the other side of the restore point and your virus programme may not look there - you think that you have cleaned everything out then it comes back on next boot-up.
I normally turn off the restore, reboot, scan and clean, delete all previous restore points, switch on restore and make a new restore point.