Im not talking, home, pro, ect. Whats the difference between the OEM and retail copies, why is OEM more expencive? If I want to use it again for another build what copy should I get?
OEM is cheaper and is usually all you need. OEM is what manufactures put on their systems. A OEM license will only allow to be installed on one computer. If you activate it on a rig, then it can only be used for that. It can be re-used as many times as you want to (clean installs). If you want to use it with a different computer you have to contact Microsoft. OEM also is lighter, meaning it doesn't have additional programs.
Unlike OEM, retail copies can be used on any computer more than once. If you build a new computer you can use that key with it. I'm guessing it would deactivate the previous install on the other computer. The point is you aren't limited to installing it on one computer. Usually it comes with additional software, that you may or may not use.
I personally use OEM, and have installed two computers with it. I figured it would deactivate the other, but both have been running as genuinely active for the past year.