This is also based on the fact that you want 'FREE' software -
There are plenty of software suites that are better than free programs that you can purchase such as NOD32 and my personal favourite WINDOWS LIVE ONECARE
Onecare is around £30-£40 if you buy it from MS site - but if you get it from an Etailer you pay around £15 and it can be installed on up to 3 machines