Well, I was trying to do this with my HP laptop. Vista is installed on drive C.
First I created a separate partition of 5 gb for Windows XP.
After that I created a new volume with the drive letter X.
Now, when I restart my computer with my Windows XP CD and when I get at the part to select a drive to install Windows XP, the only drive visible is C (Vista).
Any ideas on how to make my drive X visible ?