Hey!
I had that problem too and I was so desperate to find out how to fix it. I googled all this stuff and one thing that came up was the VoxWare Audio Codec, and you had to go through all this stuff to get it to work. Well, that was for XP. For Vista, you dont use the VoxWare Audio Codec, you use a Codec called ffshowdecoder. So basically, Its really easy to fix. Most times, when you start Windows Movie Maker, in the bottom right corner of your screen where the time is and all that, there is a symbol with two f's. Click on it, and all these Codecs will show up in a little window. Scroll down until you find MPEG. It should say "disabled" to the right of it. Click on where it says "disabled" and a little box should pop up at the top of the window. You then click the option below disabled. (I dont remember what it is) You need to do that for all the MPEG Codecs you see in a
row. If its just one or two, its not it. You can Enable all the MPEG Codecs if you want, so it really doesnt matter. If you have any questions e-mail me at
chainsawsgorawr@hotmail.com.
Good Luck!