Ah, gotcha. That sheds some light on things.
You can use anything you want for a TFTP server, including (but not limited to) Win98, Mac, Linux, Unix, Win2000, Win2k3, NT, XP 64-bit, and any of a jillion other options -- you get the idea. All you need is a standard TFTP server program to stash files on, and it needs to be reachable from the phones on your network.
CME doesn't get installed on the server; just the phone loads need to be available from the TFTP server. This can even be the router that hosts CME, from what I recall.
You may be using full-blown Cisco Unity (not Unity Express)which could be version 4. That runs on Windows 2000 Server.
No problems with any of that and Win2k3 servers.