Despite its tendency to be overprotective, Avira (for a free antivirus) is my favorite. I use it for 2 reasons:
1. It's the antivirus best rated for catching zero-day virus exploits.
2. It guards its files and your registry well once you've set a config password.
My main gripe with it is that it isn't simple to add permanent exceptions. However, when one is trying out iffy software that won't run correctly in a virtual machine, one can't be too careful. I don't necessarily agree that antivirus companies are paid to be anti-crack (I haven't seen any definitive proof), but I do know that enough information security experts recommend against installing cracks that if Avira flags it (it often doesn't), either don't use it or seek a second opinion.
DO NOT EVER disable your antivirus outside of a secure sandbox or virtual machine when testing software. The many times I've helped friends recover from infection, whether malicious or benign, have made me cautious--despite the fact that I've never been exploited by one myself.