Videos recorded in landscape mode

I recorded a really important event yesterday, holding my camera horizontally with the top of the phone to the left, and I then transferred it to my PC and a flash drive. When I played it on my computer it is sideways. I tried to edit with Window Live Movie Maker but it won't play the MP4 format videos. Does anyone know why this happens, and what I can do to fix this? I know I am not supposed to be shooting videos vertically, they look terrible.
Any help would be very much appreciated!
Staci