I used Avast for years, works great. Im trying Bitdefender now. Seems to work well.
Hope this helps,
Take care

Avast is the best free anti-virus, it has on access scanner that bassicly protects your pc ocnstantly while surfing gamming etc. and the virus scanning is very flexible you can choose a simple scan on anywhere all the way to a thurough scan of your whole pc.

It alaso scans operating memory so that none of them viruses can hide away and prevent themself running.

Get avast!

AVG is ok i suppose.

Never ever ever get norton.....