Most of the time I use Media Player Classic. There is now a version named Media Player Classic Home Cinema too.
You can find these here:
http://sourceforge.net/projects/guliverkli2/
http://mpc-hc.sourceforge.net/
The advantage of the 2nd one is that it does not require to have installed codecs to play various video files.
For DVDs, some programs give better results. I like WinDVD version 5, but in Vista it does not work as good as on XP. I tested the latest version (9) but it is not as good as the previous one, so I'll stick to the old one and to MPC.