A:
You should be fine with either one... OEM is intended to be used on a new computer with no OS (like you currently have). The copy of Windows will be locked to that specific computer, and cannot be transferred to a new one.
I believe the retail version is not as limited... so if you switch to a new PC, you can move your copy of Windows to the new machine. At least, that's how it was supposed to work in Windows 7.