I dont know that camera however usually video cameras have a facility to use an external mike. If this is the case with yours it is usual that using an external mike causes the built in mike to be turned off to prevent fead back. You should be able to put plug into the external mike socket but have no external mike connected. Doing this will give you silent videos. I Hope it works for you, it does for me. Doing this saves a lot of hassle and expense of looking for an editing program that separates the sound track from the video track.
Took a while but after a little looking at Windows Movie maker to remove/mute sound.
When you have draged the video to the time line look to the left you should see the word Video, to the right a small + sign click on this and you should see the Audio line below the vidio line. Right click on the audio line and sellect mute this will remove the sound hopefully.
What I do is click on the levels of audio button a small button to the left of the small mic. and when the levels window comes up move the slider to the extreme right and the sound in the video will not play. This is what Graham has said above.