The cheapest way to get the sound from the guitar into a standard computer without buying other gear, is through the Line In on the back panel of the computer but you will need a 1.4" to 1/8" inch adapter and the appropriate cable (I know you probably have one, it's plugged into your guitar.) Try to find a cord that goes from 1/4" to 1/8" over using the adapter though. Radio Shack might have them. After you do that you can go into your sound options through the Control Panel or your sound cards software and adjust the settings.

For an acoustic there are small mics that you can drop into the hole.