1 COMPUTER.
otherwise, explain how my vista desktop worked on a downloaded vista dvd with an OEM key?
just now i realized that.
otherwise, why would they have an oem key on the computer if its worthless?
It's not completely foolproof, sometimes it will reject the key no matter what, for no reason. Especially if you upgrade hardware. You pretty much have to keep everything the same.
Yeah, I've had the odd Vista disk which will do 4 installations on different computers, but then I've had one that will also only do one (retail) and then stop working after that, or it will charge one use each time even though it's the same computer.