Why doesn't my headset mic work, but my built in array mic does?

I bought a headset today so I can be heard clearly on Ventrilo. The array microphone works fine, but I always sound far off when I’m trying to talk to others. I want a headset so I don’t have to nearly scream everytime I want to say something.

I know the headphones aren’t faulty because I plugged them into my mom’s laptop and it worked fine. I’m plugging it into the right spot. I checked the settings and it isn’t muted. I even updated my drivers. Windows says “Working”, but still can’t pick up sound. Anyone have any clue why?

It’s a HP Premium Stereo Headset and on Windows Vista.