OK, I think I understand what you're saying. There's two elements here:
1. Hardware. Have you checked for what Spy1 said? Be sure your external mic is plugged into the mic port of your PC? If you aren't plugged directly into your PC, for example - plugged into your speakers - ensure the cables for that equipment is plugged in correctly...
2. If your hardware is plugged in all correctly, that leaves software. Since you're running Vista, you should have a little speaker in your icon tray. Right click on it, and you should three options, one of which is "recording devices."
Choose that, and you should see a window appear with the "Recording" tab active. Describe what has the green checkmark next to it....
If you double click on the device (with the green check mark) you will bring up its properties. The general tab should be active. In the middle of the window is "Jack Information." What's listed?