Many of today's manufacturers don't make their hardware drivers compatible with XP on their newer systems, so it depends on what type of a desktop/laptop you have. Once you have the make/model info, check the manufacturers website to see if they have the drivers that are tested for XP. If not, you're probably in for a world of hurt and a big headache, since you would have to put a fresh install of XP on the system, see what hardware was not recognized via Device Manager, and then test potentially dozens of drivers made for similar hardware to see if they are compatible to the point that they would be stable. I considered doing this with my laptop which came with Vista and another laptop that I have, but came to the conclusion that it was not worth the headache. If you do a google search on your model and XP you might find some folks out there (or here) that have successfully done this and will give a list of the drivers used.
With all of that said, in Windows 7 you can right click the installed app or the installer for the app and see if you have a compatibility option (can't remember the exact verbiage) and it will run through some tests to see what works.