The four bytes from offsets 0x1B8 through 0x1BB in the
MBR are called the Windows 2000/XP Disk Signature or NT
Drive Serial Number.
Please See here:
One of the ways these values are used by Windows is
to associated drive letters with mount points/disk
volumes. If they are changed, a user may find that he
can no longer log on to Windows (this happened to me),
or his drive letters may be mixed up.
To fix it, I copied the 4 byte signature (from my
backed up MBR) to the first hard drive sector by using
Linux Disk Editor (lde) from the Recovery Is Possible