Report:
After Update to VC 2.5 U3 (we had problems with joining a domain through customisation with 2.5 U2) guest customisation works with the above path.
I took the sysprep-tools from the CD1 for W2K3-Server R2 SP2 and out of the extracted SP3 for WinXP SP3.
I tested:
Windows Server 2003 R2 (Standard, German, SP2, x86 and x64)
Windows XP Professional SP3(German)
both with and without joining a domain.
worked all flawless :))