Hiya & Welcome to the site!
Personally, for all my Server systems I prefer to stay away from all the "free" solutions and go with a commercial product. Most Free AV products are only free for Personal or Non-Profit use anyway.
What your paying for with a Commercial product is REGULAR Signature Updates. By regular I mean Daily if not several times a day. Plus, you will probably only pay ~$50-$100 depending on the Quantity. Not a bad investment in my opinion. Microsoft promises to release AV built-into the OS
however were still at least a year away on that.
So, with that in mind your options are fairly broad. Here is the list of products (in order) I would recommend:
If you’re going to install AV on just 1 Windows Server 2003 box:
is a great product. It's a true security firm and their signatures are well respected.
2) McAfee Anti-Virus Enterprise
. I would make McAfee my #1 choice if you needed to centrally manage AV across your enterprise. (Their EPO product does a great job with my 5000 devices at work).
3) Trend Micro
- Always been a solid player.
4) Symantec Anti-Virus
Previously known as Norton AV. It's a grandfather in the space. I'm hesitant to list them in my recommendation being that I got burned by them a few years ago at a large Corp. I worked at. The AV engine had an exploit which was used to attack my enterprise... Not good. I'm sure it's better now. Like McAfee EPO, Symantec AV has a nice Corp. edition which allows you to centrally manage thousands of hosts.
Hope this helps. Let me know if you have any other questions!