Windows 7 - Can't invoke email client from Contact folder

I have 2 Windows 7 machines, both setup identically, except one is 32-bit and the other is 64-bit. My email client on both is Thunderbird and there are no other email programs installed.

On the 32-bit machine I can open my Contacts folder (the one in %USERPROFILE%\Contacts), right-click a contact icon and select Action > Send E-mail and Thunderbird's Compose New Message window opens with the recipient's name and address filled in.

When I try this on the 64-bit machine, I get an error message "There is no email program to perform the requested action." It suggests I install one, or, if one is installed, to set up an association in the Default Programs control panel. When I "OK" the error box, another one comes up with "An error occured while attempting to invoke the default email program."

As far as I can tell, it's already set up correctly. Clicking a mailto link in both IE and Firefox opens the compose email window. So does right-clicking a file in Windows Explorer and choosing Send to > Mail recipient. Also, if I double-click to open the contact file and click the email link on the summary tab there, the compose email window opens as it should. I just can't right-click the contact icon and open a new email directly from the context menu (saving me some extra clicks).

Again, on the 32-bit machine this works. Anyone have any clues?