Most versions of Windows use the Advanced Configuration and Power Interface (ACPI) to enter a number of different low-power consumption modes by the user or after a certain amount of elapsed time without activity. There are five different power states available to Windows on an ACPI computer:

S0: Working. This is the normal state of the computer when switched on.
S1: Suspend / Sleeping 1: The CPU suspends activity but retains all its contexts in a very low-power state.
S2: Suspend / Sleeping 2: The CPU is powered down and loses its contexts, but the memory retains all of its data.
S3: Suspend / Sleeping 3: Same as S2 but devices will need to be re-initialized at the next wake-up.
S4: Hibernation / suspend-to-disk: All contexts are written to disk in a hibernation file and the system is powered off (same as S5).
S5: Soft-off: Everything has been shut down.

The S3 state is the deepest possible suspend for the system without actually turning the power completely off. In this state, not only are hard disks and displays powered down, but power supply and processor fans are also shut off, saving even more power (and also cutting down on noise).

Desktop users who make use of the low-power states in Windows may find that at one point they are able to deep-suspend the system to S3, but then at another point they are no longer able to do so. Sometimes this happens after adding or removing hardware, but the most common reason for it is a change in the power profile, available from the Control Panel under Power Options.

If a user is certain that a given machine supports S3, but seems to be unable to invoke it, one way to force support for it is to set the "Power schemes" selector to "Portable/Laptop." This should force the use of S3 and re-initialize any system settings that disallowed it. Note that there may be other factors preventing deep suspend, such as hardware that does not fully implement it or a BIOS that implements it incorrectly, but these need to be explored in detail separately.

Serdar Yegulalp is the editor of the Windows 2000 Power Users Newsletter. Check out his Windows 2000 blog for his latest advice and musings on the world of Windows network administrators – please share your thoughts as well!