Yes, I agree : Reinstalling an OS on a computer don't change the BIOS asset tag.
I have deleted all partitions on the hard disk, and re-installed the Windows XP OS (the same OS as before) on the same hard disk.
Where was this serial number before ?
Before this reinstallation, OCS Agent was already installed on this PC. It's the serial number uploaded by OCS Agent to OCS DataBase (shown by ocsreports). AND, it's the serial number written on the DeskTop.
It doesn't appear into the BIOS.