HiJackThis is a great tool, to be sure, but it's no good without someone knowledgeable to interpret the scans that it produces.

If you do download/install and run it, by all means post a copy of the scan result in this thread and I'd be more than happy to go over it for you.

If you want to do an immediate total scan of your machine, then you can use the free
Ewido Online Scanner and see what that says.
AVG has always been a decent, free-for-personal-use antivirus for Win, but it's gone downhill somewhat since version 8 - "bloatware" is the word that comes to mind...
Avast! still isn't too bad, though, and the Comodo Firewall is pretty decent, too. (and both are free-for-personal-use, too)