Power Options Stand By, Turn-Off Hard Disk, and other don't work, help?

I currently use a personal home desktop computer running the windows xp pro operating system. The problem is that I share the computer with my roommates. And they made a bad habit of leaving the computer turned on. So I told them to shut it off when they were done using it, but kept on doing it. So I thought, maybe I should turn on the power option schemes of turn-off hard disks. To my amazement, I tested it when leaving the computer idle for 5 minutes, and it didn't shut-off. So I tried the standby mode, didn't work either. In fact, I 'think none of the power option schemes work on my computer. Does anyone have any suggestion on how to fix that & get the computer to shut itself off when left untouched or idle?