Failed SP1 update

I have an issue I've downloaded SP1 via update, and what happens is this
- Install, then get a msg please reboot your system
- System reboots and goes through step 1 and fails saying the update cannot
write some files, reboot and try again
- Reboot 2 or 3 times with same error as before then it rolls back to Vista
Ultimate ed no SP.

I don't want to reinstall completely as I have to much work to save and
backup, I have a full copy of the DVD media for both Vista ultimate and Vista
Ultimate with SP1 has anyone had this problem before or can anyone think of a
work round please.