How fast is the system?
The reason I ask is if the system is really slow, no amount of software decoding will help....the movies will skip, etc
WinDVD and others can be found on Google. A search for "free dvd decoder" might yield some stuff.
However, if you have a stack of DVDs at home, pop a few into your other, DVD equipped computer...some include free players. I know the Star Wars ones do, etc etc I was able to get those to work ok.
I've had the best luck with PowerDVD, which came with my drive.