since your computer has vista I'm assuming its a fairly new computer. you might have trouble finding XP drivers. I can't speak for HP products, but I know that other brands such as Dell and Sony are making new desktops and laptops that no longer support XP. While I'm sure that the hardware could run on XP if it had the drivers, but I have found that the manufacturers are no longer making or providing drivers for hardware.

if you can find the make and model of your various computer hardware (graphics card, network card, sound card etc...) then you could probably do an internet search and find drivers for those components. but if the motherboard is specific to that computer, then HP would be the only ones making drivers for it, and in that case you would be screwed if it doesn't make a XP driver for it.

like I said I might be mistaken, but i have had this same problem with both Sony and Dell.

If the hp website doesn't list drivers for xp in the select OS then there are none,since you don't have any drive imaging software i would use a spare hdd,remove the drive that has vista on it & install the spare then load xp & see what happens,failing that buy a comp with xp on it they're as cheap as or run xp as a virtual pc inside vista,msvpc 2007 will do the job & you can transfer data between the two

Good point although it would pay to check date of last post before replying :p