Window Live Movie Maker-can't add files

When I try to add either a WMV file or MPG4 file to a new project, I get the triangle with ! error:

Error:0xC946001E

And to "fix" I end up at a page suggesting I search for CODECs -- of course I don't know which CODEC I need, and they all warn me how it might corrupt my system.

I am not super computer-savvy so please no jargon (at least not too much) -- can anyone help? I'm just trying to load in a YouTube video, nothing special, probably not protected in any way...so that I can select a frame or two and print out a picture of it.

Vista Home Premium x64 bit

If the error is suggesting a codec you might need either divx or xvid. I have had both on my previous system without any problems however although limited there is always the possibility of system damage. Therefore it is always wise to keep your files (if not your system) backed up off of your computer.

The error doesn't suggest anything. It tells me it cannot proceed until I remove the file I just added to WMM. I indicated the error number, and what happens when I click the 'fix' link. No specifics are given on that page.