ARS Technica wrote an article about the Windows Vista OEM versions, which are cheaper than the regular versions. They explain what an OEM version is and what's missing compared to the regular Windows Vista versions, Here's a snip:
Can I buy OEM?
Yes, you can. Microsoft licenses OEM software to "system builders," which the license defines as "an original equipment manufacturer, or an assembler, reassembler, or installer of software on computer systems" (emphasis added). You can install software on computers, right?
When asked, Microsoft says that OEM software is not intended to be installed by end users. Off the record, Microsoft spokespeople have told me that the big concern in Redmond is for Joe Newbie. They don't want inexperienced users buying OEM software, but the fact of the matter is that anyone can buy OEM versions of Windows.