2. enter “regedit” into command line
3. click “allow” when Windows asks to continue
4. Find \HKEY_LOCAL_MACHINE\SOFTWARE\Clients\Mail\Mozilla Thunderbird and clik it
5. In the right pane you should see DLLPath key
6. If it has no value, mark DLLPath, right click and choose modify
7. Enter location of your mozmapi32.dll (C:\Program Files\Mozilla Thunderbird\mozmapi32.dll) and click ok.

and or

1. Click start and enter shell:sendto into command line
2. Sendto folder opens…
3. There should be MailRecipient.MAPIMail file. Probably you dont see extension, as it is Windows default setting.
4. If there is MailRecipient file, you can edit it with notepad (just pay attention to not use notepad as default application for MAPIMail file!)
5. If there is not such file, you can create one with Notepad. Just:
5.1. Make extensions visible (Folder Options> View > Uncheck “Hide Extensions for Known File Types.”)
5.2. Right click onto empty folder area and choose New>Text document
5.3. Edit file and write into it: C:\Program Files\Mozilla Thunderbird\Thunderbird.exe -compose attachment=”file://%1″
5.4. Change name to: MailRecipient.MAPIMail
6. Hopefully end of problems

You can find this solution also in: http://support.microsoft.com/kb/182378