Fran
You'll either need to convert the MVI file to something that your computer understands (eg AVI, MPG or WMV if you're on Windows), or else install a "codec" onto your PC to allow you to open that file in other applications. It is possible that your MVI file is actually an AVI file with a different name - try renaming the last part and see if Windows Media Player will open it.
You might find
http://photography-on-the.net/forum/showthread.php?t=22183 helpful...
Try running Windows Movie Maker (free
download here) and dragging your MVI file into that... if you can get Movie Maker to see it, you can advance frame by frame and Movie Maker gives you an option of "capturing" a frame to a single picture file. That's probably the easiest way to do it...
rgds
Ewan