New Tracks Are Recording Sound From Other Tracks
If your newly recorded tracks are picking up parts of other tracks, your microphone or input device is probably picking up the sound from your speakers.  

Try using headphones.  

If that doesn't work, your sound card may have some noisy circuitry.  Go to your Window's mixer recording settings and make sure that only the devices you are interested in recording are not heard.  Mute any sources such as the CD player or auxiliary that you are not recording from.  To bring up your sound card's mixer click the Window's Start button followed by clicking Run...  Type in sndvol32 and push the enter key.  

Also, make sure you've selected the correct recording source. If possible, it's best to choose a specific source, instead of something general like "stereo mix," "what you hear" or "digital wave." For example, if your recording source is a microphone, choose "microphone." If you're recording from your computer's Line In input, choose that.

To choose your recording source in Mixcraft, click on the small down arrow that appears to the right of the arm button on the track you want to use for recording. First, choose the sound device you want to use (you may only have one choice), and you'll see a sub menu with a Stereo Source option.

Again, if you already have a specific source selected, your microphone could be picking up the sound from your speakers.  Try turning off your speakers and using headphones to monitor the audio!