Windows Vista Disk cleanup. Hibernation file cleaner. 1.5GB?

[Unregistered]

Seems like a new entry to me. Anyone know what this is supposed to do?

 

[Gambler]

If you hiberate your computer, rather than sleep, a hidden hiberation file equal in size to your installed ram is created in the root of C:\ The file prevents data loss should you lose power while the computer is suspended.

If this just appeared in Disk Cleanup it probably is because hibernate has never been used before.