4. PC/laptop microphones suck. I don't know why no one bothers to test them to the same level as your average cheap dumbphone speakerphone. They pick up all kinds of system electrical noise, ...
The problem usually isn't the microphone. It's the way it's wired (per the standard) and the way the desktop/laptop is powered.
PC microphones are wired UNbalanced: They have a signal and a ground wire, rather than the + and - signal wires and everything-but-desired-signal cancelation of the balanced wiring setups typical of professional microphones.
Laptops typically use power supplies that are not grounded, so they don't require a three-prong outlet. This usually ends up with the stray capacatance to BOTH sides of the line wiring capacitively coupling equally to the laptop "ground". That means the "ground" of the laptop is at half the line voltage - about 60 volts of AC (a rotten approximation of a sine wave plus lots of other junk it picked up at an assortment of frequencies). The capacitance is substantial - not enough to shock you if you touch the laptop and ground, but enough to feel a buzz if you rub your hand lightly across a "grounded" metallic part of the device.
Plug in the unblanced microphone and hold it, put the headset on your head, or just leave it sitting on the table. The "ground" is at 60V and you are driving maybe a couple MA of it down the shield wire. The voltage drop of that current (along with any other pickup) adds straight onto your audio input. The best microphone in the world will perform horribly if hooked up this way.
Try this: Unplug the laptop and let it run on battery. Notice how almost all of the noise disappears. You can also get rid of most of the noise by tying a decent ground onto the laptop. (Unfortunately, many meetings last longer than the laptop batteries...)
Plug in a VGA monitor with a three-prog power plug, which grounds the case of the laptop via the shield and the two hold-in screwd. I've done that without actually hooking up the monitor (which would have disabled my laptop screen) by adding a couple of the nuts scavenged from another DB connector as conductive spacers so the actual signal pins are not quite into the plug. And done this on a docking station, so the laptop headset was quieted when the laptop was docked, even though I used none of the docking station features except the power input.
Make a second cable with a three-prong plug to bring a ground up to the laptop. Green wire from the third pin to a screw into or clip onto such a chassis ground point.
Or bypass the problem completely by using a USB microphone. These digitize the audio right next to the microphone proper, with everything floating at the same voltage so nothing substantial is picked up betwen the air pressure sensor and the A-D converter.