Well, as I said, a lot of guys around here will recommend MSE or Avast
It's your choise anyway. And I think you'll want to check in msconfig (go to start, and type it, hit enter) go to start up tab, usually the bad guy/s would like to activate themself when the system starts,see is there something that shouldn't be there. You may want to scan and with Hitman Pro
Just as like second opinion you now, doesn't hurt, right? http://www.sevenforums.com/tutorials/663-backup-complete-computer-create-image-backup.html
This is a very nice tutorial, for making a backup image (sometimes is easier just to re image system partition then to scan for hours, and not to be sure is everything fine or not
check and this program to: BillP Studios Download Page
If you are using firewall (you should), learn how to block a program that wants to be active online, as googleupdatebeta.exe for example.
Hope this helps,