What's the best anti-virus software that i can use for my pc?

All of these are free for home use and will work with XP and Vista.

AVG Free is the most popular free solution available at no cost to home users and provides the high level of detection capability that millions of users around the world trust to protect their computers.

Easy to use, low system resources

Automatic update functionality

Real-time protection as files are opened and programs are run.

http://free.grisoft.com/doc/2/us/frt/0

Avast-

avast! 4 Home Edition works for 60 days after the installation in demo mode. You will obtain your FREE license key by E-mail after the registration. The license key is valid for 1 year. After one year you will need to just re-register.

http://avast.com/eng/home-registration.p...

Avira-(recomended)

free-av.com 鈥?Protection and security against viruses, worms, Trojans, dialers %26 more.

http://www.free-av.com/

PC tools-(recomended)

PC Tools AntiVirus Free Edition feature highlights

Protects your PC as you are working, surfing and playing

Detects, quarantines, disinfects and destroys Viruses, Trojans and Worms

OnGuard鈩?protects your computer against threats in real-time.

http://www.pctools.com/free-antivirus/

Free spyware remover.

SUPERAntiSpyware(recomended)

SUPERAntiSpyware will remove ALL the Spyware, NOT just the easy ones!

Easily remove over 100,000 pests such as SmitFraud, Vundo, WinFixer, SpyAxe, SpyFalcon, WinAntiVirus, AntiVermins and thousands more!