Had a peculiar problem last night: Outlook Express (my default mail client) suddenly stopped being recognized as the default mail handler. It kept asking whether I wanted to set it as the default. Clicking a “mailto:” link in IE opened a zillion windows and, finally, got a message that the default mail handler was incorrectly installed or some such.
The trouble is a corrupted registry key in Windows XP: the HKEY_CLASSES_ROOT\mailto key. When you click on this key, you get all kinds of errors: cannot open, cannot edit, cannot delete etc etc.
After hours of combing the net and trying various solutions, I figured out what the problem is: the permission settings for the key have got corrupted. The cause? It seems to be an update to MicrosoftOffice (SP2 or SP3).
There is a solution: and there is no need to reinstall XP. Avoid the scorched-earth solution if you can.
What you basically need to do is to take control of the key. Evidently, it can’t be done through the registry editor — simply because the Registry Editor doesn’t allow you to do very much to edit the key or change permissions. Even if you get control of the main key, the sub-keys often don’t get changed. The trouble persists. If, for some reason, you can’t take full control of the key (see below) and its sub-keys, there’s another solution.
How to take control of the key and its sub-keys
Go to the HKCR\mailto key, ignore any error message (if you can) and then:
After each addition, give them full control of the key and its subkeys.
Back on the Advanced Security Settings for MailTo tab, click owner and check that the \machinename\adminstrators are shown as the owner.
Exit the Registry Editor.
If this doesn’t work, see below.
What to do if you can’t take control of the mailto key and its sub-keys
Fundamentally, this requires you to have more than one user account on your system, with full admin rights. I do this as a matter of course, for safety’s sake — you can always bounce general registry keys from one to the other unless something has gone really wrong.
So log out of the ‘damaged’ user and into some other user account which has full admin rights. Go off to the Registry Editor. Get to the mailto key, and add administratorS, system, and all users, including the damaged one, following the procedure outlined above. Remember to give them full control of all keys and sub-keys. Check the owner tab and make sure administrators is shown as the owner.
That’s it! You’re up and running again!