It's very easy to use, just install it while WMP is not running and when you'll run Windos Media Player next time go to Menu->Tools->Complements->SolveigMM WMP Trimmer Plugin and activate it.
From then on, you'll see a new window in the lower part of the interface where you can choose the start and end of the selected fragment of the song or video you want to create and edit.
WMP Trimmer Plugin exports the selected part with the same format of the original file, it means that if you ope an MP3 file, the resultant split file will be also an mp3 file.
It supports Windows Media Player 9.0 and higher.
21 days trial version.