i hope this isn't a stupid question, but can cardio build muscle, albeit not as quickly as weights?

After using for a month a stairclimber (up to 30 min on level 5-7), a treadmill (up to 30 min with 5-7-10% incline) - check your thighs:-)

is there anything i can do to prevent the building of muscle and just burn the fat?

Exercise builds muscle. Period. Look at a marathon runner: the women are fantasticly strong but lithe. Slow cardio builds dense muscle while fast cardio builds lean muscle.

But if you do no exercise and stick to low carb eating you'll get thin without gaining muscle.

Also if you are doing a high impact type of aerobics your bones become denser which also adds to your lean mass. Generally women don’t build big bulky muscles from doing aerobics or even resistance training and more muscle is a good thing as it burns more calories then any other type of body structure and therefore will burn more fat allowing you to become more lean. Another thing is that not all fat lies just under the skin, a lot of fat can be found internally(especially the abdomen) even inside muscle. So as you get leaner your measurement will actually decrease as you add muscle and burn fat.