To answer this we discuss "CODECS"
by R. Proffitt - 4/13/13 3:16 PM
In Reply to: Media player won't play Panasonic HD Writer VE 2.0 MP4 by KevTheAce
Both Explorer and WMP (windows media player) use a CODEC to find the length as well as decode the video. Both Explorer and WMP and incapable of finding these CODECs so you either find the one you need or encode in some way that your current setup can handle.
Frankly I rarely install a CODEC because too many CODECs have been compromised with malware. So I use VLC PLAYER instead.