Depends on what system you're running but the general consensus seems to be
Don't touch:
A.V.G. (free or otherwise)
McAfee
Norton
*******
MSE (M$'s own) is a good anti-virus for windows systems, seeing as M$ have a vested interest in fighting viruses. Having said that, I use ESET Nod32 AV and have done for years. I back that up with a twice weekly quick scan with Malwarebytes' which isn't an anti-virus software but anti-malware. Windows firewall does for me as well basically because I can't be arsed with the learning curves of other firewalls, but if I did, it would probably be the zone alarm firewall (free).
And finally a daily clean up with CCleaner for the cookies/history etc..
I've used this set up in my old XP and now Win7 with no issues i.e. They don't over step the mark by blocking anything they don't like, even the ones you know are OK and when un-install time comes they don't leave bits behind to interfere with future (competitor) AV software installs.