Today I encountered a nasty problem. I clicked on an e-mail address on a web page and instead of opening Thunderbird with a new mail message I got the following error "Der Vorgang kann nicht ausgeführt werden weil der Standard-Mailclient nicht richtig installiert ist.". This means something like "The operation can not be performed because the standard mail client was not correctly installed.".
I told Thunderbird to ckeck if it is the standard mail client under Extras -> Settings -> General -> Standard mail client (I don't know if that is correct because I have a german version); it told me that it is the standard mail client.
Finally to make it work I had to go in the Internet Explorer (7) to Extras -> Internet options -> Programs and there I selected Thunderbird as standard mail client.
Strange thing; but sometimes things stop to work without any obvious reasons :-)