Dell computers come with CD's that have driver installation on them, you seem to be missing your sound card driver.
there are a few things you can check:
1. settings in Yahoo. I installed a voice changer and it created a sound driver that doesn't read from my sound card so my mic stopped working in skype until I changed input device to my soundcard. I don't know if Yahoo has the same functionality but you might want to check the input device in yahoo settings to make sure it's your sound card.
2. Go to Start->Right-click on My Computer (Computer in Vista or above)->Manage->Device Manager->Something having to do with audio (not sure the exact wording here, school computers have device manager blocked) If you are missing a necessary audio driver there will be some sort of driver with a '!' on the icon (The word that comes to mind here is Multimedia, though not sure). If you have a 3rd-party sound card, make sure the driver related to your sound card is in the menu there.
3. Check that your microphone actually works. Plug it into another computer and try it, if it doesn't work, there's your problem.
There might be more you can check that I can't think of off the top of my head, you might want to do some looking around for it.
EDIT: Wait. Laptops. Right. You probably don't have a 3rd party sound card, so the device manager will almost cerntainly have a driver with a yellow exclamation mark on the icon.
Edited by sausage, 03 May 2010 - 11:43 AM.