OEM vs Retail? What does it all mean?

OEM = Original Equipment Manufacturer. Basically, one copy of Windows for one set of hardware. It's not supposed to be moved to any other hardware than what it was originally installed on.

You also get a shorter warranty and support (or none at all, I can't remember). Not that it matters, but you won't receive a nice colorful box full of useless manuals and ads w/ OEM either.

Retail is more expensive, comes in a box w/ manuals, can be used like 5 times before they try to say it can't be activated anymore. (Although, a short call to MS saying that you need an activation key will resolve this) Basically, you're "allowed" to port this version over to other sets of hardware as long as it is installed on only one PC at a time.

**Note: With the OEM software a call to M$, if worded correctly, will allow you to reuse the copy on another set of hardware... usually.