My windows movie maker clips keep disappearing from the video?

I have a nikon camera that I film my videos with. They come out as.mov's and I run windows so I convert them to.wmv through the program Nidesoft Video Converter.
When I upload them into windows movie maker, they are fine, but after a few seconds of putting them into the video and editing they show up as black instead of my video.
I don't know if the audio is there or not because I use a voice-over on my video so its separate. Its normally only one or two of the clips.
Please help before I smash my computer to pieces with my fist from frustration, thanks

Heres one solution: Don't use WMM
If you're willing to pay-Sony Vegas is a great alternative for around $100
There are also many other FREE alternatives online, but make sure to read reviews to make sure that they aren't viruses.
Since WMM can't even handle its own file types, thats sad. Use a different movie editor.

The quality loss is big if you convert video, . You can use one of the following software expect windows movie maker which support.mov:
AVS Video Editor (for windows) http://www.avs4you.com/AVS-Video-Editor.aspx?sct=aff&ct=regnow&cid=175028
Adobe Premiere (for windows) http://www.adobe.com/.../premiere/
Sony Vegas Pro (for windows) http://www.sonycreativesoftware.com/vegaspro

Try total video converter from www.1videoconverter.com