Rivits
if the PC is from a large manufacturer then any key you retrieve using Belarc won't work
the OS is activated / authorised by a combination of tables in the BIOS, key files on the machine, and a VOLUME OEM generic licence key
They key won't work without corresponding matching files.
Your choices are
a recovery DVD from the computer manufacturer
a full installation DVD with a new retail (not OEM/volume) key