[quote name='b0bx13']Um no. Yesterday I just installed my OEM Windows on my rebuilt comp with no issues.[/QUOTE]
It depends on the OEM windows. Some OEM versions from large computer manufacturers like Dell or HP tie their bundled OEM windows discs with the modifed BIOS of the motherboards that these manufacturers use (retail motherboards and OEM motherboards of the same model on Dell computers have different BIOS). These windows discs cannot be used on motherboards without these modified BIOSes, even if they are motherboards of the same model with retail BIOSes or BIOSes modifed by a different computer manufacturer.
Normal OEM windows discs (much like the ones you would buy from newegg) do not have these restrictions, and you could install them anywhere. However, even these normal OEM windows discs tie their licenses to the computer to which they are installed, and according to Microsoft, the "computer" to which the license is granted is the motherboard. In other words, if your rebuilt computer has the old motherboard, then there is no licensing issue. If you changed your motherboard, then you are technically breaking the terms of your license.
If you were to use your OEM windows to reinstall often (I don't know the exact number, I hear different things like 120 days between reinstalls, 3 reinstalls a year, and 3 reinstalls lifetime), your windows activation might fail, and you might have to call in Microsoft customer service for reactivation and a new key.
To make matters more confusing, Microsoft is pretty lax about reactivation. Tell them you reformatted your hard drive, and they'll give you a new key. Even tell them that you upgraded your motherboard, and most of the time they'll give you a new key, unless you get a real stickler of a customer service rep.
Plus, even if you install windows on multiple computers, it'll work as long as you got through activation without getting flagged for some reason. There doesn't seem to be a mechanism where Microsoft can stop you from installing windows on multiple machines, besides the nebulous "can't do it too often" flagging.
This doesn't mean that it's legal. According to the OEM license, you are limited in its use to THAT computer, though they're lax on even that limit.
I just like knowing that if I get a retail version, the license is for me, the user, as oppsed to the OEM license, which is for the computer. There is no equivocation when it comes to reinstalls and upgrades.