If you want to slow down the ORIGINAL then Song surgeon is good but sound gets muddy when you alter the speed too much.

IMHO WavePad Sound Editor is superior. The sound quality is maintained even when you slow it down to ridiculously slow, like 10%. Try downloading the free trial version and see if it works for you -

http://www.nch.com.au/wavepad/index.html

I haven't used riff master so can't comment on it.

Cheers