I use Grisoft AVG Free and i’ve been using it for years because it’s pretty lite and doesn’t seem to take away anything from the perfomance of the PC. I also like it because I have not had any problems with it blocking any folder shares I have at home. I will admit that they try really hard to get you to upgrade to the Pro version so you have to watch out for the little prompt messages that pop-up here and there but it has been a really solid anti-virus package for me. I highly recommend it.
You should check with your internet provider and see what Anti-Virus suite they offer and consider using it if you’re not doing anything fancy at home. for example in my area of Oklahoma the main Internet providers are AT&T and Cox cable. Both companies offer a 3-license certificate for McAfee Internet Suite.