Windows Movie Makers Help (On Windows XP) I can't change a WMM file into A WMV file! Help!?

I've clicked on Save to my Computer > Selected a Location/Name > Saved it as a WMV > Clicked Next. Then, a message pops up saying "WWM cannot save to the specified location. Verify that the original source files used in your movie are still available, that the saving location is still available, and that there is enough free disk space, and then try again." I've checked all those. Nothing works. PLEASE HELP!