i have a dat and tar file for a movie,how do i use the two together so
i can play the movie?

a tar file is usually a unix compressed file 
http://www.gnu.org/software/tar/tar.html
and here is a version that will work under windows
http://gnuwin32.sourceforge.net/packages/tar.htm

A dat file can be anything , the dat extension indicates a data file
of sorts however without knowing the header of the file (the first few
bytes in the file) it is impossible to say what type of file

You can use most popular unzip utils to untar a file... usually you
must open the app (winzip, winrar, ZipGenius) then use that app to
open/extract the tar file.

if you have Unix/Linux/BSD, the command is tar -xvf filename.tar
unless it's gzipped, then it's tar -xzvf filenamt.tar.gz

As for the .dat file, It MAY be a VCD file. There is more info on that
at http://www.videohelp.com/forum/userguides/141726.php

This is from personal knowledge of tar and the site www.vcdhelp.com,
now www.videohelp.com