I did a little searching as I was bored.
If the OS is Windows 2000/XP then NO, it appears that the only copy or way to reinstall over the MBR is from the original CD, via the Recover Console (fixmbr). So it is unlinkey in the extreem that a second copy exisits anyware on the hard drive.
It becomes a little more confusing if the OS is Win95/98/ME as it appears you can run a "fdisk /mbr" command to reinstall this from the local hard drive. I suspect however that this is just expands a file to reinstall it and is not actually already on the drive as a second copy, allthough I can't see any information about to back this up.