How to disable and re-enable hibernation on a computer that is running Windows

To make hibernation unavailable, follow these steps:

Click Start, and then type cmd in the Start Search box.
In the search results list, right-click Command Prompt, and then click Run as Administrator.
When you are prompted by User Account Control, click Continue.
At the command prompt, type powercfg.exe /hibernate off, and then press ENTER.
Type exit and then press ENTER to close the Command Prompt window.

To make hibernation available, follow these steps:

Click Start, and then type cmd in the Start Search box.
In the search results list, right-click Command Prompt, and then click Run as Administrator.
When you are prompted by User Account Control, click Continue.
At the command prompt, type powercfg.exe /hibernate on, and then press ENTER.
Type exit and then press ENTER to close the Command Prompt window.

Notes The sleep or hibernate options may not be available for one or more of the following reasons:

Your video card may not support the sleep function. In this case, update the driver for your video card, or check the video card information and supported drivers that was included with your computer. For more information, see Update drivers: recommended links.
Some settings are managed by the system administrator. For more information, see Why won't Windows allow me to change a system setting?
Sleep and other power saving states may be turned off in the BIOS of your computer. To turn on the sleep function, restart your computer, and then enter the BIOS setup. When your computer starts, instructions typically appear on the screen to indicate the key or keyboard shortcut that you must press to enter the BIOS setup process. Because not all computer manufacturers use the same BIOS, different keys may be assigned for this purpose.
If the hibernate option is missing, you may have the hybrid sleep option turned on.

for vista try:

powercfg -h off