I'm planning to buy a MacBook Pro and I'm going to need to run Windows on it. Is it possible to legally buy an OEM Windows (Vista or 7) for it or so I don't need to pay for the retail version?
NewEgg usually has these. Vista is definitely there, Windows 7 might not be there until it releases to retail (even though these are OEM/system builder versions)
Yes, you can buy an OEM licensed Windows for your MBP. However, OEM is usually restricted to only ONE machine, and the license does not allow you to transfer that license to another machine. Boot Camp, and a VM (not using the BC partition) is usually regarded as two separate machines, and hence, two separate licenses are required.