From Help and Support:
Open Power Options in Control Panel.
Click the Hibernate tab, select the Enable hibernate support check box, and then click Apply.
If the Hibernate tab is unavailable, your computer does not support this feature.
Click the APM tab, click Enable Advanced Power Management support, and then click Apply.
The APM tab is unavailable on ACPI-compliant computers. ACPI automatically enables Advanced Power Management, which disables the APM tab.
Click the Power Schemes tab, and then select a time period in System hibernates. Your computer hibernates after it has been idle for the specified amount of time.