I agree with the cleaners suggested. Here's a run down of all the anti virus/malware/spyware/ and others you should download to protect your computer.

avast! Free Antivirus= I would run this as your real-time scanner which should stop a virus download in its tracks.
Malwarebytes' Anti-Malware= run this maybe once a month.
SUPERAntiSpyware Free Edition= run this maybe once a month. It will scan your start programs, registry, and other files. Don't need to run as a real-time scanner.
CCleaner=this will clean...well about whatever else needs to be cleaned.
gmer=the above person suggested this but I have no experience with it yet. I'm going to go download it now but hey, give it a try.

ALL FREE PROGRAMS

Good Luck To You!!