Did you convert the old video files into dvd-video? In other words, are they now in a Video_TS folder as .vob files on the dvd disk? Will they play in a stand-alone player now like a commercially made dvd?
Or did you simply burn them to a disk as data files, still in their original format?
There are different editing programs for different formats. There are a lot of programs that can handle avi or dv for example, but fewer that are good for mpeg2 (or mpeg2 inside vob container files).
It would also help to know what types of editing you plan. Do you want to do simple cuts, or do you want transitions and menus as well?
One that should be able to handle most formats is Womble Video Wizard DVD. There is a free trial for you to download and test to see if it will fit your needs. It may be more elaborate than you want.