I've got a 3 year old eMachine T6212. Recently I installed a new hard drive and a new clean install of XP Pro, SP2.
I have set the Power Options to put the monitor, hard drive and pc into standby after 45 minutes of not being used.
Then, as usual, click apply and ok.
And the monitor goes to sleep after about 10 minutes. I haven't checked if the rest of the pc goes to sleep at the same time, but I know that when I come back into the room after being gone for a short time, I have to move the mouse around or tap a key a few times to get anything on the screen.
And then I have to enter the password again.
I hope you have some ideas.