if you look at the back of your computer, you should see a light blue colored hole which is your stereo line in (dont plug it into the red one,as this is the microphone hole and its not built to take such high signals so will be all distorted).
Although this is a mini plug you can get a small adapter that fits at the end of your guitar lead that makes it fit into this hole I got mine from Tesco's for a couple of quid .. then go to your sound properties, volume control then options.properties again and make sure you are recording from the stereo line in....
then it all depends what software you are using, i just use audacity because my amp lets me plug my iPod into it, and then both the backing track and guitar goes through as one signal and just records on audacity, but im sure there is proper software out there that does it better, but that's the basic principle anyway...