I know I'm late with this, but just a note that *.txt files are normally opened by Notepad. Notepad is the default application to open *.txt files, so if your Word opens them, then somewhere along the line you or someone else must have changed the default application.
Also, if the *.txt file is very large, Notepad may not be able to open it. Normally in such cases Notepad offers the use of ''Wordpad'', (usually found in Start > All Programs > Accessories).
Just something else as well. It is good policy never to open attachments direct from an email. Although good anti-virus applications should check all email attachments, it is advisable to save an attachment to the hard disk, (the Desktop is a good place for temporary storage), and then right click the file and select the option to scan with the installed AV application before opening the attachment. Email attachments are a common source of viruses.
Mark