The nonprofit organization I work for received a donation of some used PC's with Windows XP. I've been tasked with upgrading them to Windows 7 (we don't want Windows 8+ on them at this point; these are decent pre-used computers but we don't expect them to last more than 5 years). I'm a little unclear as to whether I can buy and use the OEM or not, and what the pros and cons of that would be. Since the difference in price between the OEM and the Retail is $200+, I'd definitely like to go with the OEM if I can. Any recommendations?