I found this in a few different places around the 'net - hopefully it's helpful:

Manually Put Your Computer into Hibernation

Hibernation is a cool feature in Windows XP that is 'hidden' in the Shut Down dialog box. If you have enabled hibernation on your computer, you can manually place it into hibernation. But first, it must be enabled. To enable hibernation support on your computer, you must be logged on as an administrator or a member of the Administrators or Power Users group. If your computer is connected to a network, network policy settings may prevent this procedure.

• Click Start, click Control Panel, click Performance and Maintenance, and then click Power Options.
• Click the Hibernate tab, and then select the Enable hibernate support check box. If the Hibernate tab is not available, your hardware does not support this feature.
  
• Click OK to close the Power Options dialog box.

Note: When you put your computer into hibernation, everything in computer memory is saved on your hard disk. When you turn the computer back on, all programs and documents that were open when you turned the computer off are restored on the desktop.

Here's a shortcut to hibernate:

• Hold down shift key in the shutdown dialog, then hit ENTER.
• or just hit H.
• or make your power button do hibernate through the Power Options dialog