You might want to have a look at this Microsoft KB article entitled "How to troubleshoot hibernation and standby issues in Windows XP". I've not read much myself, but had a quick glance and it contains various suggestions that cover various possibilities - although it seems mostly aimed at problems coming out of hibernation rather than entering it, they may well be helpful anyway.
Has anything changed on your laptop recently? For example:
- Do you have any new peripherals attached (even innocuous seeming ones, like say a mouse)?
- Or any new software installed?
- Potentially a recently updated device driver?
It could be something in these sort of categories is somehow obstructing the hibernation process - in which case try unplugging anything new, stop or (temporarily) uninstall any new programs, and/or try a system restore to before when the problem started.
If these activities stop the problem, you will then have to play the trial-and-error game by reconnecting devices and reinstalling software on at a time until you find out what is causing the problem.