Hi Dan,
Does the computer have any type of zip program? They should all support tar files. A tar file is just a collection of files; not really compressed. A tar could then be compressed using a zip or compress program. This one appears to have been compressed with WinZip or something similar after the files were collected together into a tar file.
See if MKS is installed by opening a DOS window and typing tar. If you get a command usage line back, type
tar -tvf C:\path\to\file\filename.tar
or
tar -tvf filename.tar
if you're in the same directory as the file to see the files inside the tar file.
To extract the files:
tar -xvf filename.tar C:\path\to\where\you\want\the\raster
The tar command is a unix/linux command so if you have a colleague who uses linux, ask them to extract the file. (the data's about 25mb, so a little big to email around)
Melita