I've used Sound Forge to edit sound files. It will change tempo without changing pitch, change pitch with changing tempo, etc. It was affordable on eBay. But there's legitimate free software that does many of the same tasks:

http://audacity.sourceforge.net/

Audacity is one of the open source projects, and is supposed to be reliable. I think it has an optional free mp3 encoder, too.

I've found this software great (1) to slow down fast licks, and (2) to place a song in a key I can play on my C or B-flat.

Windows media player will change speed, but not pitch, for mp3 and wma files. Not sure about wav files.