There's no driver for the microphone. It normally only needs "switching" on.
Vista has a strange way of dealing with sound devices, there are visible and invisible options.
Try right clicking on the Volume (Loudspeaker) icon, left click Sounds and then open the Recording tab.
Any working devices, eg microphone, should be visible.
If there are no visible devices right click in the blank area and click Show Disabled Devices and the microphone should by magic appear but shown as Disabled. Right click on the microphone and select Enable.