Ok What OS, Browser and Anti virus/spyware tools do you have.
I'm going with XP here:
Download and scan with AVG Anti-Spyware 7.5 Free
in "SAFE MODE". How to start Windows in Safe Mode
Print out the AVG Install and Scan Instructions. HERE
Be sure to check for the any updates to all these programs after you install them.
Download and run SUPERAntiSypware
, free version Home User
Now Run Windows Live OneCare
Be sure to click ONLY on the grey box below the picture of a wrench “Full Service Scan”
This may take some time especially it it needs to defragment the drive.
Please post back with any questions and results