After several years of the thing working correctly, I have developed a problem as follows:
When I press the "sleep" button on my keyboard the computer appears to go into standby mode (this is what I want). But when I wake it by hitting a key or moving the mouse it hasn't been in standby but restarts by rebooting.

I have checked the power options in the control panel and I have asked it to do the right thing i.e. standby when the sleep button is pressed.

It would be good to be able to put the machine in sleep mode , any ideas?

Thanks folks.......

Right click anywhere on your desktop that should bring up display properties.

Click the tab Screen Saver and scroll down to Monitor power and click on the power tab. which should bring up Power Option Properties, then click the arrow where it says Turn of Monitor, set whatever time you want.

Windows should go to sleep, I have mine set for 15 minutes.

Sorry for delay - This still doesn't actually do what it should.
It always used to go into standby mode allowing a quick re-start, but now it needs to reboot coming out each time which is a pain.
I have done the bits in the power options but still no good.
Thanks
Alan

What version of Windows Xp, do you have, home or pro?

What time did you pick to put Windows to sleep?

Do you have a screen saver running in the background and is it a direct 3D based screen saver?

Windows services do you have task scheduler on auto., manual, or disabled?

Do you have the box checked, for your keyboard/mouse in device manager, when you have device manager open right click on the device scroll down to properties and click on the tab power management.

Run box type: devmgmt.msc check box"allow this device to bring computer out of standby". Make sure the box is checked for your mouse to.

you say in your post"After several years of the thing working correctly, I have developed a problem". If you didn't have any problems for years, what changed .........did you add new hardware/software/or update a driver to your system?

Have you installed any new software, you might have an application running in the background which can make Windows do this, are you sharing any folders on the network because the Server service will call KeInhibitSystemShutdown which resets the system's idle timer.

What do your application and system logs say, are their any warnings, errors or information items logged.... Run box type: eventvwr

When did you notice the change?

"Right click anywhere on your desktop that should bring up display properties.

Click the tab Screen Saver and scroll down to Monitor power and click on the power tab. which should bring up Power Option Properties, then click the arrow where it says Turn of Monitor, set whatever time you want.

Windows should go to sleep,..."

The above setting only turns off the monitor display, it does not put the system "to sleep".

The "System standby" and "Hibernate (Tab)" are the two controls which govern the "sleep".

Task Scheduler has no real bearing on the issue.