Hello Everyone! Hello BSchwarz!
I got it working now, and you are right!
For everyone having the same problem as me, here's a short comprehension of what to do:
What I did:
-copy the HP-Files from a x86 Vista (\\VISTA\c$\windows\system32\driverstore\filerepository\prnhp001) to the server 2008 x64 machine
-open the printmanagement.msc
-open the drivers section
-add the x86 driver pointing to the files copied from the vista x86 machine
-add printer on XP 32-Bit via Ip-Adress
and there it is!
Kind Regards
-Stefan