I've been trying to record some guitar parts over a backing track, however after I record the guitar, the guitar track also records all of the backing track. For example if I were to use the "solo" button on the guitar track after recording a take, I would hear the guitar, and the backing track mixed up together.
 
How can I stop these two tracks from mixing?

you don't have anything listed about your gear or set up, so at this point it's a guess.
 
most of the time when you record the backing tracks along with your current track, it's because you have "WHAT YOU HEAR" selected in your soundcard's input selection.
 
this is typical and default for Soundblasters, but also prevelant in realteks or other onboard soundcards (chip).