There are a number of threads in these forums that deal with this, though perhaps none specific to the 8790. Still, the basic issues are the same.
First off, check your volume settings. Click on Start, then Settings, Control Panel, then Sound and Audio Devices. Check your volume settings by clicking on the Volume tab and then Advanced. This will bring up the Volume dialog. See if you have Microphone displayed, if so, make sure its not Muted and then click on its Advanced, make sure Mic2 Select is not selected and that Mic Boost is. If Microphone is not displayed, then click on Options and then Properties. Go through the list of devices and make sure Microphone is selected. Do this for both Playback and Recording. You have to do both independently. Go back to the volume diaglog. Check for Microphone for both Playback and Recording and that is not muted and the sound level sliders are up at least half way if not to the top. Back in Volume - Playback, you may want to try to checking Loudness in the Advanced box under Volume Control.
Once all the above is set properly, you may have solved the problem but then again, maybe not. Close out the Volume Dialog and go back to the Sounds and Audio Devices box. Click on the Audio, and click on Volume in Sound Recording. Make sure Microphone is selected and not muted and so on. Go back to Sounds and Audio and click on the Voice tab. Click on Voice Recording, Volume and again check that Microphone is Selected and has the sliders at the top. If still no go, then click on Test Hardware and go through that whole procedure. That should get your mic working.
Frankly, the built in mic sucks. It works but its very touchy, it picks up a lot of noise from the machine (being as its in the machine). You can get feedback real bad if the volume is to high or the lid is slightly less than full open. I recommend getting a separate mic, preferrably with a headset. Yeah, the sound in the headphones won't impress GS Ferrari but its okay after a couple of beers or if you are tired or your hearing isn't that good since that last heavy metal concert when you were in the front row or you just don't care all that much.
That should get you going. If not, let us know and there may be something more to try or you may have a real problem that needs Tech support.