Hiberfil.sys is a hidden system file located in the root folder of the operating system drive (in most cases, this drive is C: and to see this file you need to know How to view hidden files, folders and extension for known file types on Windows). The Windows creates this file at the system installation and its maximum size is equal to the amount of random access memory (RAM) that is installed on the computer (if you have a computer with 2 GB (GygaBytes) of RAM, the Hiberfil.sys has at most between 1.5 and 2.0 GB)

This file is used to put the system in hibernation, hybrid sleep, mode that allows low power consumption during a period of time when the computer is not used and a faster boot time and application start up after that. This is accomplished by copying into this file an image of your all opened files, documents and applications.
If your computer has a limited hard-disk capacity or your system drive is almost full and you don’t use the Hybernation mode, you can release this space by deleting (do not delete it like an usual file) the Hiberfil.sys file and deactivate the hibernation.
Solution for: Windows Vista (Home, Business, Enterprise, Ultimate), Windows Server 2008 + R2 (Standard, Enterprise, Datacenter), Windows 7 (Professional, Enterprise, Ultimate)

1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.

At the command prompt, type powercfg.exe /hibernate off, and then press Enter (if you copy this command, at command prompt use right-click and Paste).

Close the command prompt window.

To re-enable hibernation:

1. Click Start, and then type cmd in the Start Search box.
2. In the search results list, right-click Command Prompt, and then click Run as Administrator.
3. When you are prompted by User Account Control, click Continue.
4. At the command prompt, type powercfg.exe /hibernate on, and then press ENTER (if you copy this command, at command prompt use right-click and Paste).
5. Close the command prompt window.

Solution for Windows XP (Home Edition, Professional)

1. Click Start ->Control Panel -> Power Options.
2. Click the Hibernate tab, and then select the Enable hibernate support check box. (If the Hibernate tab is not available then the computer does not support it).
3. Click OK to close the Power Options dialog box.