OEM should be purchased along with a new PC, that after installed, it has to be a part of the computer, u can format and reinstall it back to the same pc, its original, legal copy. But its limitation is u CANNOT installed it again into another pc even u dont use the old pc anymore or change to another operating system. Cause it has to run a procedure to bind the copy of the windows to your hardware, i believe its call activate.

After install, u are given 30 days to activate your copy of windows with internet, where microsoft know the OEM copy u purchased is link to the hardware, so if u reinstalled to another computer, u cannot reactivate anymore thus, after 30 days, it will lock your PC.

Retail is different, u have 1 copy, u can install into 1 unit of your computer, u can move it from pc to pc, as long as u have enough license for each pc u have(if u have 5 pc running windows xp pro, u can install into 5 pc), its not bind into any hardware, so u can change, upgrade, so its more expensive. And I think u can have another backup copy running with the retail version(installed into 2 pc with 1 license, as long as they dont run simutanously), its for backup reason(1 pc down, u swap).

u can buy them saparately, the activation is the point of decision, if u activate with wrong pc, sorry, there is no turning back, the key is used, registered, for the 1st pc u activate with.

I donno the price but thats how OEM works.