Unique identifier

Hi,

I want to get the unique id of the computer which remains
same after the partioning of hard disk or reinstalling
windows (computer name may change after each instalation)

ie an id which remains same throughout the life of
system/hard disk

ie this id changes if hdd of computer is replaced

eg i want to get the serial no of hdd or processor from
within the code

or an id which reside in the rom of main board

how can i get this unique id which remains same through
the life of computer

thanks very much