How to read Winmail.dat files
If you go to your Start menu -> Programs and at the top of the list click on 'Damtp Programs', a window will open with a number of little applications listed. Start 'Winmail Reader'. Save the winmail attachment from your email to somewhere (not in the root of N: as anyone can ready your files here, n:\private will be fine), go back to Winmail Reader and press the 'open winmail.dat' button, locate the file and it will show you the contents of the attachment. You can then save the files or open then from within the reader program.
Now it is possible to set this up so that the winmail reader will automatically open when you click on such an attachment from within Thunderbird.
Double click on the attachment, you will be asked what you want to do with it. Click the browse button and type in the following into the box along side 'File name:'
Click the open button.
Now tick the box next to the text 'Do this automatically for file like this from now on. Press OK.
The reader will now open. It will open for all future clicking on .dat attachments.