Only problem is that OEM is not licensed for end users, the System Builder License requires you to resell the computer with the OEM software installed to an unrelated third party. So that seems to violate the letter of "legitimate".
Face it, XP is a product that has reached end of life. It has been pulled from the shelves and the only way to get a new "legitimate" license of XP is the one that comes with a full retail license of Windows 7 (XP mode).
If you've got it, great use it. If you don't, move on to W7.
B