Using an XP disc, keep pressing the delete button whilst starting the PC, then set the boot order in BIOS for cd first then [C] hardive 2nd, save and exit, reboot with XP disc in player and away it will go, you must format all the hardive and it ask what size partition you want, say 30 gigs for XP and 90 gigs for Vista.
once thats all done just let the disc do its job, it will ask you simple questions along the way, its easy as, then load the motherboard drivers using INI files first, check in device manager to see if all drivers are installed if they are thats all.
With win 2000, use the 4 floppies then the CD.
Put the Vista disc in and select the second partition [D] for the vista installation and do the same, then load all motherboard drivers, and printer, etc.
Its easy but a bit time consuming, but i really meant for the shop to do it for her, they can do it easily whilst serving customers.

Upgrading XP to Vista i would think you would have to do a fresh installation then load Vista upgrade [ i think not 100% sure ]