I went ahead and used the XP 32-bit and Vista 32-bit version. It installed fine and updated and scanned fine. Don't expect it to run as an anti-virus server or anything like that. You'll need to make sure all the clients attached to your 2k3 server also
individually have Security Essentials installed and updating. The 2k3 server won't update the clients and it won't run network anti-virus logs or anything like that.
Anyway, it worked great for me! I only have my and my wife's desktops, a media center PC, a couple laptops and a netbook [and a couple iPhones :-)] so having them all individually running and updating MSE works fine for me.