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Do you mean chopping entire lengths of the song out? or taking individual instruments out, while the instruments you want to keep are playing? One is much easier than the other.
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http://audacity.sourceforge.net/
Load up your song, select the bits you don't want and delete them.The only thing you'll need to be careful of is where you make your cuts.
To re-save it as MP3 you'll also need to download the LAME encoder dll, they have hand-holding instructions on how to do that on their site.
Will have to try Audacity.