I can understand you wanting to go back, I've been using Win7 for a while now and still think much of the different experience is change for the sake of it, and annoying at times. But XP is going to cease being supported long before Win7, so I'd encourage you to sort the Win7 problems out instead, if you can.
As for reinstallation, I'd suspect if you don't have an XP installation disk you would probably need to buy a cheap one from somewhere.
I'm unsure about the legalities of OS reuse. MS can be right so&sos over telling you what you can't do with what you have bought. Start quoting legal trickery about licensing and so forth. But morally if it is only running on 1 PC I don't see a problem. More of a case of 'suck it and see' I think. After all you can always claim you simply upgraded the original one. New case, new motherboard, new drives, new processor, new memory, new ......
Which brings us to the point, maybe your existing PC isn't up to recommended standards ? Why not consider a more powerful PC first rather than going backwards with the existing one ? Try posting your PC specs for the experts to check over.