NO standby option for IBM machine

Hi all,
Yesterday, I was faced with a problem I have realized that I can’t troubleshoot myself. This happened to my IBM MACHINE desktop installed with windows XP professional service pack 2 where when i click on turn off in the start menu. I only got the turn off and restart options.
But the standby option is greyed i. e.
It cannot be selected.
I Checked whether it had enough free space on the hard drive but it was enough.
Still am been faced by the problem even now.
Please help.
Thanks.
