If you use the Internet on anything of a regular basis, this is something you really ought to have considered beforehand. Not to tell you off or anything, but prevention is far better than cure.
You can get a free AntiVirus software package called AVG Free Edition from www.grisoft.com . There are other free packages available, but this is the only one I've tried for myself.
Also, it sounds like you might have spyware on your PC, so head to www.lavasoftusa.com and
http://www.safer-networking.org/en/index.html to download Ad-AwareSE and Spybot Search & Destroy respectively (both are completely free programs for home use, Ad-Aware has a free version available) or search for them both on www.download.com.
Install all of that, use the in-built updating programs to get them up to date and then scan your system with all of them and hope for the best.