XP should have been installed FIRST, I know XP has a boot manager built in, dunno about Vista..but everybody i have seen on tech forums always installed Vista AFTER XP was installed.
If you just want the shit Vista has from Media Center just install Media Center only.
Then ya got lots of extra HDD space for Lesbo porn
Better yet install Media Center and Ultimate Ubuntu..only is ya don't have a ATI card installed.. with ATI and Linux
Forget that..ya may fuck that up, ya may get a little confused where the drive and swap file should be on the install.