Avira have a basic AntiVirus that is available for free for non comercial purposes. This would be perfect for Students or Home PC's.
The client is very unintrusive and i have installed this on dozens of dekstops and laptops with no ill effects what so ever.
There was already a post in regards to free 6 months but I've found that I have never needed to go to the professional version.
The are no forms to fill in and the only advertising that you get is whenever the AV does an update to get the latest virus definitions, you get a message asking if you want to go pro.http://www.free-av.com/