- OEM - Only allows you to do a clean install
- Retail - Allows you to do an upgrade of existing OS
- Volume - Also known as the corporate version
The 5 first digits of the Pid says what license it is, and the last 3 digits says what license cd-key it accepts.
- OEM - Pid=82503OEM
- Retail - Pid=51882335
- Volume - Pid=51883270
Note that one cannot change an OEM/Retail version to a corporate version by just changing the Pid. There are other differences in the files included and you will still have to activate it with a corporate key.