Sure, it's my default video player too (I love it) (I use Aimp2 for music though). Btw, you should use the Home Cinema version of MPC which, I believe, is updated more frequently, and use one of the recent beta versions as they are all super stable, ime anyway. Here's the link to where you can download
MPC-HC beta versions. They are updated almost daily, so I just update every couple of weeks or so. It's super easy cause they allow you to install a new version over the older version and to retain all your settings and customizations.
Anyway, to make MPC the default player, open MPC or MPC-HC, and hit O which will open the options menu, then open the "formats" tab on the left, and check the boxes for each and every file type you want MPC to be the default player for. Hit OK and/or apply and that's it. After that, when you click on those file types you checked off, MPC will automatically open and (hopefully) start playing them.
A
short cut is to look below the file extensions box in the same Formats dialog and just click the
Video tab and hit OK and/or apply and that should take care of it for you.