Windows Only Programs
AVG - I use this on some computers and it has worked well. Try it out.

Avast Anti Virus - It is free for home users. Let me know if you use this - I would like to hear your experience.

Avira Free AV - Again, let me know if you use it.

BitDefender - they actually have both Win and linux versions for free.

Comodo Anti Virus -  This looks like a very nice free anti virus solution.  It looks like they are offering it free to businesses as well.  How nice.

VCatch - Yet another choice for your free anti virus needs.