There is no difference between the OEM version and the Retail version of Windows (In features anyway) But I'm pretty sure that with OEM copies must buy a piece of important hardware (Motherboard,CPU and etc)
Once you install an OEM copy, it is tied to that mobo. You can switch HDD's and other periph's, but if you get a new motherboard, you'll have to call and see if you can get them to give you a new key. Usually they will, but it's not 100% sure. As far as support, no, you can't call microsoft and ask them questions. But have you ever done that?