The hibernation feature in Windows, first introduces in Windows 2000, enables a computer to save its state in a hibernation file on disk. State in this context means the contents of the computer’s memory at the time of hibernation. The hibernation file, hiberfil.sys located on the boot volume, is the same size as the installed memory in a computer.
For portable computers, and maybe workstations, hibernation is a great feature, but probably not for servers. A server is not a machine you typically want to hibernate. As stated above the hibernation file is the same size as the installed memory, so if your server has 16 GB of memory you will see a 16 GB hiberfil.sys on your system volume. This is a big vaste of space for something you probably will never use. So how to disable the hibernation functionality and remove the hiberfil.sys file?
Windows Vista introduces a great new command line utility called powercfg.exe which enables you to configure every aspect of Power Management on a system. By using powercfg.exe with these parameters you can disable the hibernation feature and remove the hiberfil.sys file:
powercfg.exe /hibernate off
After this command has been execute Windows will remove the hibernation file automatically.
No all server systems enable hibernation, so you wil not always see the hiberfil.sys on all systems. Remember also to turn on the ability to see system files in explorer to be able to see the hiberfil.sys file.
This info is also documented in this KB article:
How to disable and re-enable hibernation on a computer that is running Windows Vista