How do you mute the sound on a video clip using windows movie maker?

To be more specific so you guys understand. There is this part of the video where my friend is saying something confidential and I want to mute that part where she is saying that. Basically the video is 3 minutes long and at 2 min and 20 seconds is the part where there is confidential information. I just need to mute it for 3 seconds at that part and thats it. And then I can play the rest of the video clip with audio.

Can anyone help me with this question? If it is not possible to mute video clips can anyone tell me if there is any free video editing packages out there? Links are appreciated (: Thanks x