Something doesn't sound right with your install as my versions of DP have nice clean usermusic folders, however i will check them both (full and demo).
About the CPU, if you click outside of the game window or alt tab from the game it should use 0% cpu. When inside the game i agree it does use 95-99% (at least for me) and this is something to look at.
Hope you continue to enjoy it and make that purchase
If we can figure out these things then expect a new version shortly. Might be after xmas for these particular things but theres already another demo and full version coming soon with some fixes.