Tews is correct as of Vista; however, the Windows 7 license changed a bit.
It is now illegal to use an OEM key unless you are planning on selling the machine to someone else.
If you are doing a clean install on a fresh-built computer without any OS on it, you should buy the full retail version.
If you have a previous qualifying OS (basically any Windows), you can use the Upgrade license.
It is possible to do a clean install with an upgrade version: Clean Install with a Upgrade Windows 7 Version