Yeah, avast! is free. Just search it exactly as it is, so: avast!
The first (non-ad) link should be the manufacture's site. Go to the downloads section, and download the latest version.
After that, I'd suggest doing a full scan of your computer to clean up any shit you might have.
(There's there time of scans you can do on avast!. There's the boot-time scan, which scans your system REALLY throughly at startup/reset; then there's quick scan which runs a quick scan on your computer; the other full scan checks all your computer files.)
I recommend a boot-time scan for more obvious malicous viruses, trojans, etc. The other two should work fine in any other occurence.