Service Pack 2 install fails during "backup registry"

I recently reformatted my hard drive and reinstalled Windows XP Pro. I have
been installing Windows Updates, but I can't get Service Pack 2 to install.
It goes through the process of backing up files, but once it reaches the part
where it attempts to backup the registry, the installation wizard closes. A
window pops up that simply says the setup has been canceled due to an error.
It doesn't give any specific error.

I have updated my video and sound card drivers, run checkdisk on the hard
drive, and defragmented the drive.

I've never had a problem installing Service Pack 2 on any computer before.
It installed just fine the first time on this computer before I reformatted
the hard drive.

Any suggestion would be much appreciated!

Thanks,

Jill S.