I am on a budget and building a PC, I want to get the OEM version of windows but I want to know the main difference between it and the normal version. Will I get a license key and CD? Can I change hardware like Hard Drives, processors, or graphics cards? (I will not be changing motherboard). What are the main differences?
The main thing to note is that an OEM license will be tied to the computer you install it on. You can not transfer an OEM license to a different computer. Upgrades other then the motherboard are okay, and if you do have to swap the motherboard you will have to call Microsoft for reactivation, and its not guaranteed that they will.
I suggest going with a full retail key. You may not think you will upgrade your motherboard at first. But the day might come that you need to, or the day might come you want to build a brand new computer. With a full retail key you would be able to move your OS license to the new computer.
Second thing if it matters to you is no support with OEM license. I don't typically care about this, but I know some people do.