hiberfil.sys - hibernation file
Windows operating systems contain a single file - hiberfil.sys - which is used to store data from RAM from open applications when the PC goes into hibernation mode. The hiberfil.sys file makes it possible for the system to be restored to its active state very quickly.
In Windows 7 the file should be found at the root of the C drive. It may be several gigabytes. Windows 7 compresses the hiberfil.sys file and by default it's set to 75% of the PC's RAM.
You can remove this file by running the following command. Be sure to right click on cmd and choose the option to run command prompt in admin mode.
powercfg.exe -h off