There is another way which I always encourage to use!
Are you a gamer, @geoff ?
I use ToneLib Jam, as everybody knows by now 
It has a Rockmith like interface. It’s very easy to slow down songs here, though under about 70-80% speed the audio starts sounding s#cky!
ToneLibJam is free to try and often costs about 40€ discounted:
Here is a description how to get the songs: HOW TO: creating a GP5 with synched audio from Rocksmith PSARC songs
You just need to follow the DOWNLOADING SONGS part - it works directly in ToneLib. Ignore everything else in that post!
This is what Low Rider (free download) looks like in Tonelib:
Main edit view
Score view
Rocksmith-like 3d Game view
Different speed selections
You can see the notes you play highlighted in realtime and synched to the original audio.
Oh, and it has some effects built-in. It’s not VST and those are not the greatest ever. But fun!
Examples for other songs:
(Scroll down for prepared videos!)