I have used something called Tplayer on a windows 98 system this would auto cue and auto mix tracks. Worked quite well.
Winamp can do something similar.
Have you tried googling for auto dj or similar.
Here is a link for one http://tristarautodj.sourceforge.net/
And a bit about how it works here http://www.ghacks.net/2011/04/13/aut...xing-software/