How can I play DVD on my pc?

I've changed the windows on my pc and now I don't have the programs I used to have. Today I tried to play a simple DVD ,it's a cartoon, and windows player won't play it; I installed RealPlayer and it asks me to upgrade (and pay) to play dvds!! The files on the dvd have .VOB and .BUP extension. Can someone help me choose the right program for this dvd?