DVD movies are generally in a totally different format than regular video files, and a bit harder to work with than your normal video files. Most video editors deal with “computer” video like .AVI, .MOV, .WMV or .MP4 instead, so you may need to use something like Handbrake just to get it into a file format you can deal with. And like any other multimedia editing, it’s a lot faster to do on a fairly up-to-date computer than on older ones due to the sheer amount of computing required to transcode something.