I also want to add TDSSKiller to the recommendations.

http://usa.kaspersky.com/downloads/tdsskiller

Rootkits are small bits of malware that are extremely stealthy and difficult for antivirus software to detect. They in themselves don't do much damage, but they download other damaging malware and hide their presence as well. You may remember around a year ago I caught some malware. I tried removing it with every bit of software I could, but it couldn't take care of the problem. My antivirus software wouldn't update, and many functions didn't seem to be working (like the realtime protection, which even showed that it was off, but I couldn't turn it on). When I finally ran the above little program, it found several rootkits, and suddenly my antivirus software found loads of viruses and began functioning again. It took a long time to uproot everything, and I still wasn't sure it had worked (some rootkits are literally impossible to detect). I ultimately restored my computer to factory settings, just to be safe, but I was able to back up my important stuff, and only because I had used TDSSKiller was I able to do what I needed and without so much worry. Fortunately it appeared that nothing had been infected that I backed up. Give that little program a run or two or three. I think there are boxes to check for extra thorough scanning, so I would give the program a basic run, then check the extra boxes and run it again.