Built-in mic won't work with skype? 10 points for correct answer?

All right well.. I have a Dell Inspiron 1545 laptop. It has a built-in microphone. Last night, I was on skype on the family laptop (which is the same type of laptop) and the mic worked fine. Then I switched to my personal laptop, and got on skype. On my laptop, The mic wouldn't work. However, I know it has something to do with skype because if I webchat on AIM the other person can hear me. Also, if I use an external microphone (headset) then Skype works just fine. Can anyone tell me what's going on and how I can fix it so I can use my built-in mic?

Please be specific. For example, don't say "Adjust the microphone settings" tell me how to adjust them. Anybody who helps me fix it gets 10 points. (Also, I'm pretty sure the mic isn't muted on skype because I checked, but maybe there's a setting or something I missed)