How do we convert computer files? URGENT!!!?

So we're doing a project in which we have to use movie maker. We have filmed everything we need for the project and after we uploaded the files from our camera to our computer and got ready to open them in Windows Movie Maker, it is telling us that it does not support those types of files. No Windows application will open the files. We have even tried using two different cameras and two different memory cards. We're lost and we do not know what to do. Our files are currently in .mov format and we preferably need them to be in .avi. Any help is welcome. Thanks in advance!