If you have got as far as the activation, you need to ring Microsoft on the number given and explain you have changed your PC. You are not allowed to run the same copy on two PC's.
They key is generated by a computer at Microsoft, as a result of a "hardware hash," a long sequence of numbers based on a information found in your computer's hardware. The specific devices used are as follows:
Display adapter
SCSI adapter (if found)
IDE adapter
Network adapter MAC address (if available)
RAM amount range
Processor type
Processor serial number
Hard drive
Hard drive volume serial number
CD-ROM, CD-RW, or DVD-ROM
This ensures that one person's key will not work on another system.