Yes they can. Altough, it depends:
* If you have Windows 2000 SBS (and only then!) you need an optional installation disk to activate the extra licenses. That disk is operating system dependant, so make sure you order one for Windows 2000!
* In any other case, only the license for Windows 2003 Server will do just fine. (so witout any need for installation disks)
Greets,
Jente.