It depends on the quality of your sound card, i think cubase, cubasis or sibelius (or any other high quality recording program) will have some software bundled with it that can detect external imputs from a guitar or some other instrument, but it is quite hard to set up (we had them set up on an imac at school, it was pretty cool, you played a few notes and they came up on the screen). There is also a program called "Guitar Rig" that is made by a company called Native Sounds, which attempts to turn your soundcard into a guitar preamp, but again you need to have a really good soundcard to make it happen (i tried it and got heaps of lag and really crappy sound quality
). You'll also need a 1/4 inch to 3.5mm RCA converter or something like that.
That being said, for all the hassles of setting all that stuff up, it's much easier to just get a program like powertabs or some sound program to play guitar sounds that you just tune to...OR to just buy a cheap tuner for like 25 bucks
