Everyone makes good points here. I personally use a laptop. I store my KMA's on an external (x's 2 for backup). Beavis is right, if your hard drive ever fails and you don't have a back up, reinstalling all those songs will be a monumental task and if you have a weekly show, you'd better be prepared to sling cd's around again or your screwed. I personally keep a spare laptop with me at shows too so I have a contingancy for the computer too. (much easier than carrying my cd's around as a contingency). My Sony Viao has been very reliable up to date, and I've never in two and a half years had to break out the second computer. I think another point that will save you heartache is to limit the use of the computer you use to host karaoke with, to doing just that and you'll have many fewer problems in the long run.
Also, never upgrade to another version of Software the day of a show; give yourself some time to get accustom to the changes and deal with unforseen problems. Good luck!