Which anti virus is best?
What system are you on—Mac or PC?
if it is a 32-bit pc than you can go for nod-32, its lite and does its work well otherwise kaspersky is also good enough but a bit heavy.
I used AVG for years, still on a few computers of mine, but did get a virus on my laptop.
I've moved to Kaspersky (not free unfortunately) and it seems to be pretty solid.
There's a number of good, free AV packages available before having to resort to paid packages. AVG, Avira, Microsoft Security Essentials, and Avast.
Then, to offer some protection from malware in general, use spywareblaster and spybot's immunize feature. Those block known IPs, domains, cookies and other elements from infecting your PC through your browser.
Use symentic its really good works for me