There are more options...
What Mobo do you have? Some (Asus, MSI etc) have option of selecting boot drive at startup (press F11 or something like that).
Other option is to get XP to boot using a floppy. Just needs a few files copying onto a floppy although you may have to edit the XP boot.ini file on the floppy (google search should bring it up). Put the floppy in when you want to boot to XP, leave it out when you want to boot to Vista
Kryten