Whenever i had to dualboot i shoved xp on got it all up to date then installed vista
So, In your case unless i'm mistaken
Sort partitions (dunno if you can in vistas own gubbins with vista business ?)
When you do sort partions, Easiest is google etc for installing xp after vista, Follow the found guide/s and bobs your uncle
No doubt if i'm wrong someone will come n have my guts for garters in a following post