With OEM-versions the Serialkey only works once, as far as I know. Whilst you get to reinstall a Retail-version several times.
Actually, you can re-use OEM serials up to 10 times, but you must do so on the same machine (i.e. you are reinstalling Windows on the same computer because of a problem). The catch though, is that you cannot use an OEM serial on more than one computer, nor can you transfer the key to another computer if you remove that OEM copy of Windows from the original machine.
Basically, if you install an OEM copy on to a computer, that serial number MUST stay with that computer. With a retail license, you can transfer the license to a different computer, but you can only have the retail license install on one machine at a time.