Verily, you should know what to protect your pc from, not have a program tell you
yeah for lazy asses that don't want to clean their mess up everytime (with like Combofix, CCleaner, GMER, manual registry edit)
there is Avira Free. work wonders. i set it up by default on every customer's pc. or Windows Essentials Security Package.
Avast and AVG are heavy as trucks, norton too. NOD32 and Avira are really light euristic based antiviruses... never had issues. while AVG did a disaster 2 years ago by deleting a false positive windows core dll.
kaspersky is not bad too... but anyway with a modem-router with integrated firewall, you really don't need any antiviruses unless you surf warez/pr0n sites... but there is ubuntu for that