Most people who run Windows are accustomed to spending $50 or more a year on anti-virus software. But if you use Windows at home - not for a business - you can lower that cost to zero with one of a few free anti-virus programs. Try, for example, Grisoft's AVG Anti-Virus Free Edition, which has coexisted well with both Windows XP and Windows Vista. Alwil's Avast! 4 Home Edition is another good option. Free anti-virus protection also is available on a Mac with ClamXav, but without any OS X viruses for this program to catch, its primary utility may be stopping you from accidentally forwarding a Windows virus.