search for a file called nohiber.txt, atleast win98 had this one.
I think that all components must support hibernation for it to be avaliable, try unplugging everything and see if you can choose it. Also play around with the bios and the settings for desktop, under the screensaver tab there is a button at the bottom (!) wich you also should check up on. I dont got the english version of xp so I dont know exacly what it says but have a look and you'll probably figure it out.