I tried to hibernate my PC, and got a message about insufficient system resources. Since then, my hibernate option has disappeared from the shut down menu, and there is no for hibernation in the power options where it usually is.
XP Professional SP2 with parts of SP3 (some fixes etc.)
Update - Cleaned up disk, now have >5GB on a system with 2GB RAM. Still no hibernation option, but can set laptop to hibernate when battery gets to a certain point, or when I close the lid. This appears to put the computer into a standby mode.
EDIT - selecting Hibernate as an option when the lid is closed will default to standby after the power options window is closed.