Sleep mode - Computer does not got to sleep

My system is a Dell Dimension 9200 running Vista Ultimate.

For an unknown reason my desktop computer does not go to the sleep mode. I
did set the power options settings for the computer to go to sleep after 30
minutes.

One odd thing is that the display does not turn off either. I set the screen
saver to turn on after 15 minutes - which it does. I also set the display to
turn off after 20 minutes - but it never turns itself off.

Any idea what could be wrong?

Go to Advance Power Settings.
Set hard drive to turn off a minute or 2 before sleep.
Set Hybrid and Hibernate modes to "Off"
Under USB settings select "Disable"
Under Multimedia select "Allow ....."

The hybrid sleep feature and the hibernation feature in Windows Vista may
become unavailable after you use the Disk Cleanup Tool :
http://support.microsoft.com/kb/928897

Thanks for the suggestion. Unfortunately it did not solve the problem. I also
checked Bob's suggestion with no success. A note - I suspect it may have
something to do with the monitor because it does not turn off. I set the
screensaver to start after 10 minutes and to shut down after 15 minutes. The
screensaver starts as set but the monitor never turns off.

Am using ultimate also,
Unable to find these settings; Set Hybrid and Hibernate modes to "Off

What options do you see for Hybrid and Hibernate?