The only thing I could think of is to get yourself a
direct box, or some sort of jack to turn your guitar's TRS line into a headphone jack... get the music on the computer, and I think you can mix in the track you're adding in Windows Movie Maker. Ideally you'd want to properly mix the audio, but unless you have software like Cool Edit or Sonar that'd probably be hard to do :/.
Does your camera have a microphone jack? You *could* try buying a Y-jack and connecting your amp and whatever you're playing the music on directly to the camera itself. If you have two microphones lying around, you could mic your stereo, then mic your amp and hook the amp mic directly into your camera.
The tricky thing to deal with in either case is your levels... too loud, and it'll sound crackly and distorted, and if it's too quiet you can't hear it, so you'll have to do some experimentation. But yeah, the best/ easiest way I can think of is to get your audio onto your computer somehow, and mix it into your video while editing.
