Tried a lot (for free).
Avg Free (older versions) used to be good. Newer versions are a complete system hog and there is little help available.
Microsoft do a great free AV program, namely Microsoft Security Essentials (MSE). Takes up far less space than all the rest. Have tried it, it works fine, but there are no options to "fiddle" about with.
Avast! has got to be one of the best out there at the moment. For a free program it covers everything. It's new version 5 is a bit strange (for version 4 users), but it does the trick. Go with it!