Avira Antivir Personal Free Anti Virus is a great piece of security software that handles protecting basically all versions of Windows from viruses. It is very lightweight compared to commercial products like Symantec and even F-Prot, which I have both used in the past. The only draw back to this software is that it updates randomly each day, which could be a bit of a pain if you are busy doing something. Avira offers other security applications on their site, as well as commercial versions of their Antivir Anti Virus software.