I do this very often. There is a local aluminum extrusion factory that uses PCs to control all their various machines etc.
Because they are running on propriety software they must have XP on them.
I just order a generic Dell and then wipe em out and put XP Pro on it.
So far Ive YET to come across a PC (Vista or Windows 7) that I couldnt put XP on.
Some of the drivers are easier to find then others....but it can be done and Id guess with the age of your PC quite easily.