Get Avira (the free one) and if you want to spend some money, buy SpySweeper + my personal favorite Norton 2010.
CHeck your processes tab, (ALT+CTRL+DEL if you didn't know), and look for any suspicious processes.
If you have found any processes you're like WTF'ing at, write their names down, look them up at the ******net, and if something isn't right, stop the process.
Otherwise, boot up in safe-mode, without internet, and let all of your antivirus scanners run.
If that still doesn't work, reformat your HDD's.
AND IF YOU'RE STILL FUCKED, PUT A NEW WINDOWS ON YOUR PC.
If that doesn't work, shoot yourself in the head.
Otherwise, if you got teamviewer, pm it to me, and I'll help ya out in an hour.