My main PC doubles up as my media PC, got it plugged into my 42" plasma. I use Vista Codec Pack and the x64 components, along with Media Player Classic Home Cinema edition.
KMPlayer is another good player with its own codec pack, meaning you don't need to install any codecs your self. I've always found KMP to handle HD files very well plus you can get some nice skins for KMP. Good to keep on your system if ever you need to play some random file type but don't want to install a codec pack just for that file.
MPC is always my main player for .mkv's and .avi's ect..