Follow this link and get Winamp, the karaoke plug in and the key change software -- it's all free.
http://www.sax-n-dotty.co.uk/download.htmThen you either need to rip karaoke discs with another free program such as Autograbber or buy downloads but be aware the karaoke manufacturers aren't real thrilled by people format shifting their cdgs. You definitely need to be 1:1 with a legitimate track on a disc for every file you put on the computer.
Or you could avoid all the potential legal hassle and buy an RSQ player - they'll play mp3s, cdgs, dvds, whatever.