Johnny is right .dat files (DATA)cannot usually be read even by notepad - they`re all gobbledygook. Are you sure its a .dat file you want to open? Is it in fact a word document with the wrong extension? If so save it to disk and rename it with the right extension.
I think the boss has sent you a system .dat file - God knows why. What was he trying to send you? An Office Document? What does he expect you to do with it? Did he mean to send you an attachment at all - it could just be spurious.