T&G,
Had a bit of the same problem a couple of months ago. Got an excellent response from this web site:
www.windowsbbs.com - its free and got some excellent material on it.
I have cut and pasted the replies I got and it solved my problem. I presume that you are using XP of course.
Open 'My Computer' > 'Local Disk C:' > 'WINDOWS' > 'system32' > 'CatRoot2'
Delete the items in the folder 'CatRoot2' (Do NOT touch the first 'CatRoot' folder!)
Reboot and reinstall the updates
or
There are 2 'CatRoot' folders - do NOT touch the first one. Open the second one named 'CatRoot2' and you will see 2 folders and 5 or 6 files. Delete those folders and files, reboot, reinstall the updates and you will be back in business.
For what its worth
Cheers
(edited by me to get the URL right)