When I click on an email hyperlink in a browser window (IE7), I get a Windows
Internet Explorer error message that says: "Could not perform this operation
because the default email client is not properly installed".

I have two machines with nearly identical software and settings on each one.
This problem just cropped up on only one machine. The other is fine... I
can click on an email hyperlink in IE7 and my email client, Outlook Express,
pops up.

I have been through all of the OE settings on each machine. They are
identical. I have followed EVERY step on this page (
http://support.microsoft.com/?kbid=306098 ) to make OE my default client
(even though it always has been and has been working fine up until today,
when I noticed this inability to send mail from a link in a web page). I
have tried all three methods of registering OE as default shown on that page
and I also checked the registry to see if I needed to add a data string to
the default mail client key (instructions at the bottom of that page). The
value was already there.

I have also opened Folder Options|File Types in Windows Explorer and checked
the values for URL:MailTo Protocol. The value reads as it should...
application: "C:\PROGRAM FILES\OUTLOOK EXPRESS\MSIMN.EXE" /mailurl:%1
And the path to msimn.exe is correct.

Otherwise, OE is functioning perfectly for me. I can also use the Send To
feature with no problem.

Does anyone have an idea how to fix this?