After a Microsoft Windows XP-based computer returns (or "wakes up") from hibernation, it may enter hibernation again after five minutes of inactivity, regardless of the settings you have configured in the Power Options tool in Control Panel.
The computer's BIOS performs a full boot while it returns from hibernation and clears any wakeup signals that might be active. Therefore, Windows cannot determine why the computer woke up. Windows then returns to hibernation after five minutes of inactivity to avoid certain situations. For example, Windows may then return to hibernation to prevent a program from waking up the computer and causing the battery to drain while the computer is not being used.
This problem may also occur in Windows Vista. After you wake a Windows Vista-based computer from hibernation,
the computer may enter hibernation again after two minutes of inactivity.
For more information, click the following article number to view the article in the Microsoft Knowledge Base:
You may experience problems after you resume a Windows Vista-based computer from sleep or from hibernation