Were you using an account with Administrator privileges ?
Or it may be that you need to take possession of the registry entry, Right click on the Key and select Permissions. You should see a list of Users, select the Administers group and see what permissions they have, ensure they have Allow checked against Full Control. Now see if it allows you to change the values.
If none of that works you could try rebooting into safe mode and see if that will allow you to edit the values.