ACPI S3 mode
"Sleeping" is a feature of the computer BIOS. If Windows XP does not provide an option to "Sleep", make sure that ACPI is enabled in your BIOS.
Hibernation
S0
Check drivers for the chipset and especially for the video card.
In S1 (sleep mode) only the hard drive and monitor are shut off, everything else runs normally.Page 112 in A+ Hardware Managing, Maintaining, and Troubleshooting.
S4
No. Sleep mode is a low power standby mode. No processes are taking place while in sleep mode.
compatibility mode
sleep mode.
The operating system
Sleep mode.
Sleep mode uses less electricity than running mode.