SuperUser has basically the same question (http://superuser.com/questions/2/free-antivirus-solutions-for-windows/), as well as a discussion of the costs of choosing free over paid.

There was some discussion of (c) and (d) on the spambayes mailing list a couple of months back that you might want to read if you do this. I started work creating a "leave spam on server temporarily" option (based on header classification, which looks like it would work ok), but then got caught up with other things. You're welcome to that code if you want it (ask on the spambayes list).