Vista - Microphone Detected but not working

Hi here,

I've had my Dell XPS 720 with Vista on it. I'm trying to get the microphone on my logitech headset to work. I haven't had any trouble with it as of now. I've been using it to talk on ventrilo, in-game in steam games, but all of a sudden, it stopped working. I haven't changed any of the settings or anything. The microphone hasn't been sat on or stepped on, so it's in fine shape still. In the sound manager, Vista detects the microphone as being plugged in - as a "Microphone/Line In - SigmaTel High Definition Audio CODEC", but no program notices any sound coming from it. It is set as the default microphone and it's enabled. The sound comes out of the headphones, but the microphone does not do anything, but it is detected and is said to be working in the "recording devices" tab. Is there anyone that could help me get this working?

Specs: Dell XPS 720, 2.4 GHz Intel Quad Core CPU, with 4 gigs of ram. Thanks for the help, I appreciate it.

Many thanks,

Phillip Oh