Assuming you're using Windows 7 Media Center:
To Allow for recorder to sleep when idle and wake and record shows
- Disable 'password on wake' in power settings (control panel)
- Clear the password for default logon account
- Set 'Sleep After 2 hours' in Win7 Power Settings
- Set MCE to start automatically on system boot (in MCE Setup)
- Set 'Boot on power - (or equiv) in system bios so when power fails and UPS shuts system down, and power is restored, windows will boot and MCE will load and scheduled recordings will continue
- Run gpedit from cmd prompt and go to: Computer Config, Windows Settings, Security Settings, Local Policies, Accounts: Limit Local Account use of blank pws (change to disabled) - This allows Remote Desktop to Media Center with no passwords, since the password had to be blanked for un-attanded entrance into Media Center from sleep.
- Make sure that there is no media playing when you are not around; in order for the computer to sleep in 2 hours
This and a few other decent Windows 7 MCE tweaks can be found here.
You will/may also want to check into Windows' advanced power configuration options for "Away Mode":
Away Mode is a Windows Vista feature that is designed for media PC scenarios. Away Mode enables the Windows Vista PC to appear off to the user, when the system is actually operational and processing background media tasks such as recording television content or streaming media to other devices.