On my office PC I'm running Windows 7 with automatic updates turned on. It seems that recently it updated Windows Media Player (WMP) and when I tried to play an AVI file, instead of my usual player of choice (Media Player Classic installed with K-Lite Codec Pack) WMP popped up and offered to configure itself. It showed a list of different file extensions which it could play and some of them, including AVI, were checked by default and were impossible to uncheck! Moreover, when I ran Media Player Classic and asked it to be the default player for AVI files it couldn't do it anymore: the system returned WMP to be the default AVI player. That was horrible because I can't stand Windows Media Player. So I looked for a way to uninstall it, but it's not in the list of programs to uninstall. Luckily, everything's not lost yet because there is an easy way to disable WMP without any registry hacks.

When you open Control Panel and choose "Uninstall a program" there is a link called "Turn Windows features on or off" on the left side of the window.

There Windows Media Player can be found and disabled. I preferred to disable all "Media Features" because I don't need DVD Maker and Media Center on my office PC. After disabling WMP I could change back default handler for AVI and other video files to my player of choice, not Microsoft's one.