What is hiberfil.sys

hiberfil.sys is a file the system creates when the computer goes into hibernation mode. Hibernate mode uses the hiberfil.sys file to store the current state (memory) of the PC on the hard drive and the file is used when Windows is turned back on. In Hibernate mode the PC power is down entirely, so you can even take the battery out, put it back in, and be right back where you were. hiberfil.sys is a hidden file. It means that you could see it in windows file manager only if you checked 'Show hidden files and folders' in Folder Options. 

hiberfil.sys is Windows system file and you can’t simply delete it as you would for, say, a file that you yourself created but no longer wanted on your computer. To delete hiberfil.sys in XP: go into Control Panel –> Power Options –> Hibernate tab. Uncheck the box, reboot your PC, and then you can delete the hiberfil.sys file. For Windows 7 or Vista run command line utility as Administrator and type “powercfg -h off”.

Note: Any malware can be named anything - so you should check where the files of the running processes are located on your disk. If a "non-Microsoft" .exe file is located in the C:Windows or C:WindowsSystem32 folder, then there is a high risk for a virus, spyware, trojan or worm infection!