I found this in a few different places around the 'net - hopefully it's helpful:

Remove Hibernation File

If you do not use hibernation, make sure you do not have it enabled, which reserves disk space equal to your RAM. If you have a hidden file on the root directory of your C-drive called hiberfil.sys, hibernation is enabled. To remove that file:

• Go to Control Panel, select Performance and Maintenance, Power Options, Hibernate tab, and uncheck the Enable hibernation box.