I think we're going to end up need another sticky on this topic because I constantly see such horribly bad advice as "just open the m2ts file". Look, that's great, but, clearly if you're recommending such bad advice, you've never heard of or seen seamless branching titles. What do you do then, keep loading them in what you believe to be the right order?
If you want a freeware solution, google for MPC-HC and ffdshow. If set up correctly, and there are great guides to help with that, you can open the blu-ray from a folder or mounted ISO. You can even open the MPLS file directly. This gives you chapters and a way to play movie only...for free. If you want menus and special features, grab yourself a copy of ArcSoft TMT3, PDVD10, or WinDVD 2010. All have their pluses and minuses.