new op system

definition of retail is that it isn't tied to the hardware like OEM is.
thus, if you change motherboard/such on computer you just reinstall and activate it again (unlimited times, supposedly) While on OEM's it really isn't allowed/doesn't work (unlimited times, yes but only on the original hardware)

Also since that gives you only one license, you can really only have it installed on single computer/hard disk at a time. (not that anyone will be any wiser if it's installed on 2nd hard disk that isn't used) the problem mainly is that it can be activated only on one computer at a time and if you do 50 activations/week, microsoft will most likely get annoyed and block the key from being used.