Both desktops are running W2K with SP2. Although I've set the Power Options to Stand by after a certain time, they just don't. I'm also using Vice Versa for backups and if I've put them on Stand by manually, they will switch on and back up at the appointed times but don't power down again. Incidentally, I'm on a wireless home network - could this be part of the problem?
Could very well be, although that doesn't happen with me. Any active foreground applications that are running from your system tray for example or in the background can activate your computer from standby. Just make sure all your applications are closed or not doing any thing.