Problem with Guest Additions

[Prototypjack]

Hi,
I've finally got my new vista-machine.

I'm trying to get a WinXP running with Vista as Host (Vista is Home Premium and XP is Home). It works perfectly as long as I avoid installing the Guest-Additions, if I do so the machine asks me to reboot (completely normal) but afterwards when the xp reboots it tells me that there's a problem with the registration data of that os and there is no way to get into it. (The OS was registered before the Adds are installed!).

Anyone got that before?
I really don't know what to do because the Adds are kinda essential for me.

Thanks in advance & Best Regards
Max