One thing I hear rather often is that “I can't install X because it won't run on 2003”. If you're talking about most simple sorts of devices (i.e. mice, scanners, keyboards, gaming peripherals, etc), chances are you're just being fooled by the installer.
Driver installation works something like this - There's one or more inf files which describes devices and their driver files. Windows understands this inf file, and copies the drivers whereever they belong. The setup program just does this for you.
So, when you get told that the drivers are incompatible, here's the trick: If it's a self extracting sort of thing, just right click it, and extract it manually, WinRar and Winzip both do this. Inside, you'll find all sorts of stuff, a folder with a name like Win2kXP is likely to be in there. Inside are drivers for 2000/XP, and will more than likely work with 2003.
Step 2 here is to plug whatever dongle or device you want to use in. When Windows tells you its found new hardware, tell it you'll find your driver yourself (so don't search for you), once you get to the screen where you can pick all teh different sorts of devices Windows supports, click Have Disk, and browse to your inf file. If necessary, you will have to choose which device you want to install. Click next, finish, and you should be good to go.
Sometimes the inf files will be hidden on the CD somewhere - just search it for *.inf, or poke around in folders like “drivers” or “support”.
G. Andrew Duthie reminds me to note that somtimes there is in fact a reason that you shouldn't try to use drivers for 2000/XP on 2003, or 2000 drivers on XP, etc. Pulling the above stunt with something like a SCSI controller is a very, very bad idea. Conversely, pulling the above stunt with something small like a mouse or keyboard is probably not going to break anything (don't ever try and install mouse adn keyboard drivers on the same reboot, if the mouse and keyboard conk out, you're S.O.L.). So, use judgement if you try to do what I do, or you may very well be practicing your Windows installation technique.
If it's an MSI, you'll need to crack it open with something like Installshield (or the SDK maybe) and find the platform requirement condition(s). Just delete the condition all together, or fudge the numbers a bit (Windows 2003 is v5.2.3790, that's major 5, minor 2, build 3790). Service packs are metered in increments of 128, starting at 0 for the initial RTM release.