I prefer AVAST FREE than AVG FREE and MSE.. Avast is lightweight, so it doesn't affect much on your system performance. Tested all 3 AV's via Virtualbox, Tried prevention, detection and removal of Zero-Day Malware test.
AVIRA FREE is also good but i hate the way it gives you a nagging screen all the time. Hasol.
For Free AV, choose AVAST or MSE(Microsoft Security Essentials)
For Paid AV or IS, choose ESET NOD32/SMART SECURITY 5
But in the end, nara jud na nimu tanan, if kahibaw ka unsa imu gibuhat sa net, dli rajud ka ma virusan. Dont watch too much pron.