This one might fall into the "Duh, Marc" category, but when I get stuck, I figure maybe someone else out there might get stuck, too. (And it makes me feel better to think that way.)
I was trying to get Skype set up on my Dell Latitude D820 laptop this morning (which is running Vista), and I couldn’t get the microphone to work. When I went into the Control Panel, Sound, and looked at the Recording tab, I saw this:
So the internal microphone was there, but it was "Currently unavailable". After a little poking around on the Web, I found the answer: I needed to set the Internal Mic as the default:
Once I did that, all was right in the world again.