Yes you can. I did it just a few nights ago. I just used Partition Magic in XP and made an NTFS partion of 30GB for Vista x64. Although you can probably use the partition manager in Vista 32bit and alot about 20GB instead. (I just like to get my money's worth out of my software)...I already had XP 32bit and Vista 32bit installed. I did however remeber to Ghost a copy of my system beforehand as I have read of an issue with not being able to boot into your system afterward. Trust me that was not the case. Always back up your system regardless. Anyway, you'll probably want to have a Bootloader such as EasyBCD or Vistaboot Pro on Vista and XP to easily manage the OS order of priority, to change the name of and to tell the difference between the two Vistas when your system boots up.

Order of my install: XP 32bit, Vista 32bit, Vista 64bit, you may want to hide the two Vista Drives from each other like I did. Before I did so I found that a IE 7 download defaulted to my Vista 32biit drive's Download folder instead of the correct DL folder in Vista 64bit which I was using at the time : o

Siince you have Vista 64 already installed and I have ignored the core of you inquiry for the majority of my reply,(next time I'll read it in full) I would go for XP 32bit so you will have a zero compatability issue version of windows to work with. If you still have you original XP disk then all you have to do is call Microsoft to get a new key(read: at no cost) I have done it for Vista when I switched hard drives and it would not allow me to use my original Vista key when it asked me to activate my system again.

Hope this helps you and if not somebody else surfing for an answer.

Hello friend i under stood u r problem
its awell godd configuration .
to begun with dual o/s u must install lower version of os and go then higher version
step 1
install xp prof (or) 64bit xp pro
u may also install both the 2 o/s in any order
step 2
install vista or 64 bit vista in another drive in any order
its complete
ok
IMPORTANT ISSUE
once u install the both o/s
the higher version takes the drive c
suppose u install vista in d,e,or h
it will show as c drive
and i lower version the vista installed drive should show the original name
byeeeeee
if any doubts u may contact my mail ..... byeeeeeee

Its also possible to run several op systems on the same drive without ever having to partition and lose any files by installing a virtual drive by using software like vmware this runs as a seperate drive totally seperate to your main drive and is ideal to run a safety backup of either a existing drive or enables you to try a new operating system without touching your existing one so you can try say unbutu and still have vista incase you dont like it

Thnx You so Much..............^_^