Hello FrankSwaby,
Are you able to log into a different user account and duplicate this issue? Have you always had this issue or has it recently started where you can't make any Windows feature changes? Were you able to duplicate this issue in a Clean Boot?
New User Account:
http://windows.microsoft.com/en-us/Windows7/Create-a-user-account
**** **** To help troubleshoot error messages and other issues, you can start Windows by using a minimal set of drivers and startup programs. This kind of startup is known as a "clean boot." A clean boot helps eliminate software conflicts.
Try to perform clean boot and check if it helps, here is the link:
http://support.microsoft.com/kb/929135
Note: After you have finished diagnosing, follow step 7 in the article to boot to normal startup.