Totally Free Antivirus?

I just installed AntiVir on a spare computer that I give out as a loaner. Even though it was free the software is showing an expiration date of 11/30. So i'm assuming this is only a limited time trial version, even though it doesn't say that. Am I wrong? and is there any antivirus out there that is free forever, with no subscriptions for updates, ect? I refurbish a number of computers for low-income people and I'd like to have them with some kind of antivirus, even if it's not the best. Free is a must. Any ideas?

avast! 4 Home Edition
http://www.avast.com/eng/download-avast-home.html

I liked avast it was just too confusing for me: P

So AVG for me too.