A rootkit is a possibility, or it could be a run of the mill malware. Try to eliminate the most obvious things first though, and follow drone's advice (don't forget to work disconnected from the net, and in safe mode if necessary). If nothing comes up, or even if it does, this info is from an old post I made some time ago, to save alot of retyping:
Quote Originally Posted by Caravel
You'll probably need to download hijackthis, and post up a log here, this is time consuming.

First go to this post and download whichever of these programs that you don't already have and run full system scans with all programs, except SpywareBlaster which you don't need yet, and fix any problems found (AVG Antispy is more manual requiring you to set which problems you want to fix. Don't fix anything you're not sure about. Before you run any scans, go into all of your browsers and delete all of your temp files and cookies.

https://forums.totalwar.org/vb/showp...2&postcount=21

After this you'll need to download HijackThis from here (ignore the commerical ads on the site):

http://www.majorgeeks.com/download3155.html

Extract HijackThis.exe to C:\Hijackthisxyz\

Rename HijackThis.exe to Hijackthis1991.exe

Run HijackThis and do a full scan and save a log. Post up the full log here, don't do anything yet. Note: You can seriously mess up your system by deleting the wrong thing from HijackThis. HijackThis is not an anti-spyware scanner. Not everything in your log is spyware, some things are critical, so you need to be careful and not delete anything until it's been positively identified.
Follow that to the letter. Let us know how you get on.