I needed to change the access rights to the system log on two computers (both running Windows 7), as described in this question. I did it successfully on one of the computers.
On the second one, I started regedit.exe as administrator from the context menu. It did not ask for a password on start, although I was logged in as a non-administrator. I added the user and groups I needed to the access list, then I clicked "Apply". I got an "access denied" error message with just an "OK" button, and there was no UAC prompt where I could have given an admin password (or possibly a domain admin password - one of the accounts which needs access came from the active directory) to resolve the issue.
When I log in with my administrator account, the same thing happens - regedit starts, but it will not let me add user permissions, not even for local accounts.
What could be causing this, and how do I add the users?