If reinstalling Windows Updates doesn't make any difference, update virus definitions and scan thoroughly for viruses.
Download and install Spybot search & destroy and let it check and fix problems. click here or use other spyware removal software such as Adaware.
The problem may or may not be caused by virus/spyware, and even if it is, the above programs may not detect them, but try it first anyway.
If problem persists, download and install the standard version of "Hijack This" click here and let it scan. If you know what to fix, tick the boxes to let Hijack This fix it. If not, click "Save Log", then copy and paste the text on the log file here for others to have a look.