Well, windows media player bundled with windows 7 should work well for standard movies. For blu ray movies you will need a player that specifically claims to play blue ray, but most of these cost money, and I do not know any that are free. I've only had experience with cyberlink powerdvd: No. 1 Media & Blu-ray Player but it looks like it costs 100US$

You might search google for blu ray players, but honestly cyberlink is the cheapest I've found