Open iTunes and somewhere in it's settings you should be able to set it to
"always check" that it is the default player for those files. This is an additional
setting besides being the default player.

Open WiMP and set it to stop being the default player (and to stop
checking that it is) for those same types of file.

For the best performance and direct integration with iTunes dump the
version you downloaded from the BearShare web site (the more recent versions
were crippled to please the RIAA) and get the version in this thread:
Sticky: BearShare has been sabotaged.