I have seen software that comes bundled with some sound cards have karaoke options in them. You can check your control panel at Start Button->Control Panel to see if there is a Control Panel Group for your Sound Card and check the options. You may need to buy a new sound card that does support karaoke, but unfortunately I do nott remember the model of card I saw with this option.
Here is a link to a karoake forum whose users might have more experience with your need: http://www.krazykats-karaoke.co.uk/forum/
Good luck and post back if you have more questions.