Al4x
You need a minimum of two partitions into your HDD, but I'd advice you making 4 to have a layout like this:
c:\ the boot partition which should be active and primary. You don't need to make it big, just about 1Gb or less to keep the boot files.
d:\ first partition (logical into an extended) to keep one of your OS, probably XP since it's the first one you must install.
e:\ second partition (also logical into the extended) to keep the second OS, in your case the Vista thing once XP is installed and working.
I'd strongly advice you to make another partition f:\ to keep your files, the ones you woldn't like to lose in the case you needed to format any of the OS partitions for XP or Vista. You can direct there your documents, pics, mp3, etc.
Rgrds