you can install it but as i said you will
NEVER be able to Boot into XP.
read my experience with that NTLDR.
here ->
Installing XP NTLDR error
so if you really want XP, then you have two options,
1- get rid of the Recovery Partition. - it'll be a big pain in the back side

, if something goes wrong in Vista.
2- Ask the Registry guru's here for hiding that Partition from "XP Setup". - doin this very thing is a big pain in the back side


btw why do you wanna
De-Grade your system to XP now.???