Easy.

First off, the file is equal to the amount of RAM you have in your computer. 2GB RAM results in a 2GB hiberfil.sys

hiberfil.sys is the Hibernation File used for when your computer goes into Hibernation. The contents of RAM are copied into this file and the computer powers off completely, rather than a sleep state where the computer goes into a low power mode.

To remove the file safely, go into the Control Panel / Power settings area, and disable Hibernation. Reboot, and the file should disappear.

Another option, for Vista and Windows 7 users, is to open a command prompt, type: "powercfg /h off" and reboot.