Need a legit copy of XP Pro or 7 Pro

[FaceAche]

With the course i'm doing, you build a PC from scratch, and then get the option of purchasing that PC. It seems like a pretty good deal for the price, so i've bought my PC

The problem is that it doesn't have an Operating System with it.

So i'm looking for a legitimate, genuine copy of either Windows XP Professional or Windows 7 Professional.

Would anyone know where i could get one without having to spend hundreds of dollars more?

I don't really want to put Linux on it, but that might ultimately be my only option. Ubuntu perhaps.

The PC seems to have some really good features on it, HDMI, Optical etc, so i'm thinking of using it as a Home Theater machine. Any thoughts or suggestions?