Microsoft Knowledge Base article 49500 lists about 25 "independent vendors" of AV software products. They make no recommendations, and don't say that they all make "Windows Certified" products, but it does give you a place to look if you're not satisfied with what you have.

That article has a link to "Certified" AV products to work with Mickey'$ Server systems.

There are a w!h!o!l!e! b!u!n!c!h! of freeware, shareware, open source, etc AV programs around as well. Some of them appear to be pretty good; but without a vendor to take the heat if it screws up, you have to do your own evaluations.

Fortunately, there are some official (and some unofficial) organizations that keep an eye on the threats, so that it doesn't take "vast commercial resources" to keep up with the most common nastys.

John