No Sleep Option

I just did a clean install (RC1) on a new hard disk in a computer that
previously used XP (with the ability to put the computer to "sleep"). On
this computer, in Vista, there is no sleep option anywhere. (The power
button offers to turn the computer off; "sleep" is greyed out on the fuller
list of options (such as restart). Under "Power options--advanced setting"
there is no option to sleep, nor can one define the power button to do
anything other than shut down, and the sleep button has only a "do nothing"
option.)
In the BIOS, Power management/APM is enabled and all the other ACPI
settings seem correct. In "Device Manager--System Devices" it shows as
working properly settings such as "ACPI Sleep Button," "Microsoft
ACPI-Compliant System" and "ATK0110 ACPI Utility."
Any ideas as to what the problem might be?