u got 2 options if u dont want to use WMP ,
First , To uninstall it ( as i far know u cant just disable it ) , to remove it , go to start > control panel > add/remove programs > Add/reomve windows component > Un-check WMP and follow the steps.
Second, if u want to play ur movies and stuff with some program other than WMP , for example , if u wanna play ur movies with real player , then right-click the movie file > open with > choose real player from the list > AND check the box saying : always associate ....etc......
Now,wn ever u play the movie it wont open WMP again.
Hope that helps