First, go to Control Panel > Sounds and select the 'Recording' tab there.
Now look at the list; there will be at least a 'Line in', and a 'Mic'.
Which hole in the computer is your kit connected to? If your microphone is plugged into the 'Mic' socket then select that as your default.
(There might be another 'Mic' too if you have a built-in one on you computer.)
If it is a USB device then that too should show up here.
When that's done, go to the 'Volume' icon near the clock on the bottom right and right-click on it.
Select 'Open Volume Mixer'.http://img401.imageshack.us/img401/7162/px38.jpg
On this screen, you can mute any channel you don't want to hear and control the volume of the ones you do.
Does that help?