WavePad is pretty basic but good software for music junkies. If you don't like parts of a song then take it out. Want to do mash-ups like on the radio all you have to do is select the part you want then select the part you want it to mix with the click "edit" -> "paste mix". it's really cool. The program itself, i think, is open source, so if you want to change something then you can.

This is the second program i tried to mix songs together. The first was Audicity which is availible from www.audicity.sourceforge.com it seems almost exact except i wasn't able to paste mix the songs let alone paste bits of other songs onto another song.