You are only allowed to use an OEM version if you are an OEM and building PCs for resale. You can't even legally do a straight install, there's a kit you need to get from MS for building your distribution version of Windows, and you have to sign an agreement with MS and become a business partner to get the kit.
MS personnel have stated that, while various tricks for using upgrade copies, or using an OEM copy for your own use may (or may not) function now, there is no guarantee they will keep on working.
Buy an upgrade copy if you have a valid OS to upgrade from, or buy a full retail copy. Or buy use Linux and get the Microsoft monkey off your back.
