Advanced Configuration and Power Interface (ACPI) is an open industry specification that defines a flexible and extensible hardware interface for the system board. Software designers use this specification to integrate power management features throughout a computer system, including hardware, the operating system, and application software. This integration enables operating system to determine which applications are active and handle all of the power management resources for computer subsystems and peripherals.
If you are not sure whether your computer is ACPI-compliant, check your computer's documentation.
ACPI is not always properly installed during the installation of Windows OS.
There's a simple test to check whether it is properly installed. Click Start->Turn Off Computer. If StandBy is available in this menu, then windows is able to work with ACPI. If StandBy button is grey and you are sure your PC supports ACPI (almost all computers manufactured since 2000 do so), you should reinstall windows. An important tip: before reinstalling go to your BIOS setup -> Power management and set there ACPI to enabled and Suspend mode to S1 (important!). After installation completes, you should set Suspend mode to S3.
