Jahn, your needless transcoding made me shiver
I'm sure the difference you got from this transcode was negligable but here's how to stop you being useless with MPC.
i) The easiest option, just install foobar2000 .. Its a superb player, one of the best technically. It's also free!!! After install it will ask you what associations you want to give it (what files do you want to play with foobar2000) .. simply select "Nothing" in the drop down.. and thats it.. you have foobar2000 installed but it will never run unless YOU run it!!
Download it from here
http://www.foobar2000.org/
The special version includes lots of extras for a bigger download (still a few megs only) and is the recommended version to install.. you get a nice ABX tool installed then to be able to do blind tests with.
Thats it.. run the program, drag the mpc file onto it.. press play.. it just works (and will do with ALL formats)
ii) if using winamp then simply download the plugin, listed on this page
http://www.musepack.net/index.php?pg=win
Close winamp if running... install.. rerun winamp.. drag and drop mpc file on winamp, it just plays
iii) if using another player i'm pretty sure there will be an MPC plugin for it, sometimes the winamp one works.
Basically just use option i)
every windows head-fi-er needs to have foobar2000 installed, even if its not their default player, it really is the swiss army knife of audio apps.. your dbpoweramp is good for that too, but foobar2000 is clean, and wont touch the rest of your windows if you don't want it to.
Use this to enjoy the mpcs as they are