Problems Exporting (Windows Media Player encountered a problem while playing the file)

When I exported a file onto my desktop, Windows Media Player played 75% of the video and then abruptly stopped and said "Windows Media Player encountered a problem while playing the file."

When I tried it again, it did the same thing, but on the second time, it wouldn't play the file at all.

The first time I exported the file, it was over 6GB.  The second time I exported the file, it was only 89MB.

So there is a definite problem with exporting the file into MP4 format.  How can I fix this?

Thanks!