You don't have to change any BIOS or OS settings to run Media Center. Double-click on the "Media Center" icon and you're good to go.
I guess one thing to double check is the power options. You should make sure the CPU power saving options are turned on. They are turned on by default, though. When you get your TV tuner card, you can have the computer go to sleep after a set time. It will automatically wake up to record scheduled recordings and go back to sleep.