Have you heard of DivX?
Try downloading the latest version of any DivX player off Kazaa with all the decompressors (don't worry, the decompressors usually come with the download). 99% of the time you'll get the right decompressor. DivX players play almost anything.
Lol and not to worry about the slow computer thing. My second computer is a (wait for it..) Pentium I 166MHz
with only 24Mb of RAM. It still runs all movies fine. So if the ".MPEG" extension doesn't work, try the original file you d/l.
Hope that helps you.