Vista or otherwise, it can be a challenge and a half getting microphone stuff to work with any computer. Things to check (you’ve done some of these, but I’m being complete for later reference):
Line level or Mic level? There are two specifications for microphone (well, audio) input, and your microphone and card have to match. Many audio cards have two inputs, one for each level. If you put a Mic-level input (most unpowered headsets) on a line-level port, you’ll get a very, very, very soft input (inaudible unless you’re shouting). This is a common problem with Macs, which typically have only line-level inputs and need special microphones. Some laptops do, too. The rest of these apply pretty much only to Windows.
Right card? Lots of motherboards have more than one set of audio inputs. This isn’t likely on a laptop. I’ve even seen cases where interference from one set of inputs causes a very light “ghost” signal on the other, causing you to think you’ve got them connected right.
Gain? Many cards have the option of doing a 20 dB boost on the microphone. Check the sound control panels and see if you can turn it on.
Muted? It’s possible on most cards to independently mute the microphone at the card (via a software control on the system) or the Mic itself (usually a switch on the cord somewhere).
Drivers installed? You might need separate input and output drivers.
Software screwed up? Most audio cards come with not only drivers, but a whole suite of sound software, usually including things like reverb effects, equalizers, and media libraries. These often have their own set of independent, overriding controls for volume and mute, and are in my opinion almost always worthless. You might have several varieties competing with each other.
Multitasking? If you’ve got run-all-the-time music players and the like, they sometimes take control of the audio even when they’re in the background. Try killing them.
All this fails? Get a USB headset and switch to USB audio. It has some downsides (slight lag, performance hits, usable only on the computer), but USB audio systems usually are just plug-and-play.