No, they are compatible, unless you access something very specific on
Windows Server 2003 that isn't on XP. I can't think of a specific example,
but it's just as compatible as XP is with Windows 2000.
Probably some of the stuff in System.DirectoryServices wouldn't be
compatible. Mabye some cryptography. But mainly, I'm thinking I heard
somewhere that the ActiveDirectory integration with is much better on 2003
server. Or was that with Whidbey? Hmmmmmmmmmmm.