Is winmail.dat file a virus? Why does this happen?

Below is a link to a post from Microsoft as to why this happens and what you can do to stop this from happening in future

http://support.microsoft.com/en-us/kb/278061

Does the winmail.dat file have any content?

Not all winmail.dat files have content. Right-Click the winmail.dat and open in notepad. The first 2 lines of text will give you an idea if it contains any relevant data (eg: IPM.Microsoft Mail.Note)

How to open a winmail.dat file?

Found a simple and easy to use website that helps you extract the contents online

http://www.winmaildat.com/