im not sure if this is it, but this may your answer:
when i reformatted my HD and reinstalled XP (2 days ago), it showed up again, after not being there before my reinstall.
that leads me to believe that one of the "hot fixes" that you download from MS on help and support may disable that feature.
it just might be better not having it... but i can't say for sure that this is your answer (its just an educated guess)