You could look at this technical bulletin on Windows Product Activation. Judging by your comment, you probably don't want to go to these lengths, but maybe it can provide you with some inspiration...
WPA generates a hash based on the serial numbers of some of the following hardware items:
- Display Adapter
- SCSI Adapter
- IDE Adapter
- Network Adapter (MAC address)
- RAM amount range
- Processor type
- Processor serial number
- Hard drive
- Hard drive volume serial number
- CD–ROM / CD-RW / DVD-ROM
