[Stuart]

It could be that the Windows Update datastore has been damaged (it happens, from time to time).

To get around this do the following:

1. Right click on "My Computer"
2. Click Manage
3. Double click "Service and Applications", then "Services"
4. Find the service called either "Windows Update" or "Automatic Update". Click "Stop" to stop it.
5. In "My Computer", go to your Windows Root folder (usually C:\Windows)
6. Rename the folder "SoftwareDistribution"
7. Go back to the Computer Management window, and restart the service.