Open the Volume Control (should be in Tray, otherwise open Start Menu, Programs, Accessories, Media Tools, Volume Control or Start Menu, Run, SndVol32) and select Properties, check the Record radio button, press OK, and make sure the microphone is selected. Also in Properties, enable the Advanced controls and turn the Mic Boost on and the AGC off. Use the Mic Volume slider to adjust the Mic.
If the new Mic takes batteries, make sure you have good ones in it.