There is loads of antivirus software out there, ranging from stuff that has loads of adware built in, is watching your every move and reports hugely exagerated results (ahem SystemDoctor) to stuff that costs you £300+.

For the home market I've tried numerous antivirus software products and I've found one that's free and I'm happy to stick with. Avast Antivirus from Alwil Software.

All you have to do is download the home edition from the downloads section of the Avast website. (http://www.avast.com/eng/programs.html), register your email address and they send you a home license code. Presto!