Since for whatever reason I'm not allowed to edit my own post, I'm adding this information here. I've also tried changing MPC's settings, but to no avail:
Okay, since there are many questions on this topic, none exactly the same as mine, I'd like to make myself perfectly clear. I know about Default Programs, and I know about Autoplay settings in the Control Panel. What I want to achieve is use Media Player Classic (comes with the K-Lite codec...