If you type in "independent anti-virus tests" you'll see av-comparatives; I've been using their results for a while now to determine the most consistent top performers. Personally I use Avira, been a bit lazy to check if Avira is still a top performer, but hasn't caused me issues so far.
For Windows: Kaspersky - Excellent Nod32 - Almost as excellent as Kasp Norton - pants. AVG - horrendous. For Linux: Custom methods..
I'm using Avira, couple of years ago I use AVG and norton but still my computer crushed and with Avira I haven't experience any crush except on the hardware.
Kaspersky internet security. Even though it have some bug issue when you have multiple download with rapidshare, megaupload and similar services.... It seems I'm unable to download after a few download from those services. Have to pause my protection so I can continue to download or restart my notebook. Kind of sucks but it's still worth of the protection. ;(
i use mcavee until now and always update every 3 month... its free if u ever heard smadav...its not bad trust me
if you want free antivirus use avg/avira anti virus... but for me eset nod32 is the best antivirus..(for me)