Even if the movies are already in digital form on the TIVO, to the best of my knowdlege your only option to get the video out of the unit is with a frame grabber (if you had ReplayTv it would have been different) . MJ is really out of the picture until you have recapture the videos and saved them in a compatiple format.
You could use a pure frame grabbing card ( e.g ViewCast Osprey ) or buy a tv tuner that also supports frame grabbing (e.g. ATI Tv Wonder, Hauppuge WinTV ... ) or you could go for an all integrated multimedia card (ATI AllInWonder). Make sure that you get something that supports s-video since the quality of the video capture will be significantly better.
Once the card is installed and configured, there are several tools that you can use to capture the videos. Do a quick search on video grabbing on Google and you'll find many options. MJ also has some capturing functionality, but I haven't used them much yet so somebody else will have to fill you in on those
Claudio