I use a program called Audacity. It's free on the web somewhere. Search the forum. I can change the tempo (not the speed) down to -50% and have it sound okay. Okay, enough to transcribe most stuff anyway. Much slower than that it sounds mushy. I hear Transcribe software is better.
Some stuff with distortion and lots of background is just tough to transcibe.
Just to clear, when you change the speed, you change the pitch and the tempo. You just want to change the tempo and keep the same pitch. Thank goodness software will do that.
(Makes me wonder how they did it in the old days? Slow the 45's down to 33 1/3?)