Media Player wont play videos!!!!

Basically i have always been able to upload videos from my mobile to media
player fine! Then a couple of weeks ago i got an ipof and downloaded itunes
and quicktime player! Just went to upload a video today and it wont play on
media play (It will on quicktime) but it has to be a media play file format
to work on a powerpoint presentation!!! I really need it for Coursework and
it keeps coming up with the following messge 'Windows Media Player cannot
play the file. The player may not support the file type or might not support
the codec that was used to compress the file' What does this mean??? Help how
can i make my videos play in media player i have already changed the film
format but still doesnt work!!!