I have used AVG for many years - the paid for version (Full Internet Security Suite) It is personal choice for me. Many like AVG - many Don't saying that it slows things down. I don't find that and I am on Windows 7 Professional 64 bit.
The latest thing seems to be MSE (Microsoft Security Essentials) click here together with Windows Firewall. I haven't used it so I can't comment on it (MSE) but it appears to be liked by many who answer queries on here saying that it runs quietly in the background.
Those that do use it will no doubt add their comments for you.