I struggle to explain this one. This is a duplicate setup - i have two systems very nearly identical, both HP DC7600's, full-up 4GB RAM, 160GB HD, fairly fresh Windows XP Professional SP3, typical applications: MS Office, Picasa, iTunes, Realplayer, Google Chrome browser, blah blah. Nothing exotic.
One of them, after being left with a half dozen tabs open in Chrome, and not much else going on other than AVAST Antivirus running in background, upon checking back in with the Screen Saver "On" [starfield], no password needed, the screen/display freezes on the background image, and nothing else pops up on screen except the mouse cursor. The system appeared to be alive and well but 'invisible'... I could plug things into the USB ports and hear the familiar 'blip' audio that alerts to what the device is, etc, but nothing visible ever showed on the screen. Tried to invoke Task Manager and got nothing. Tried Ctrl-Alt-Del twice in succession, no reboot. Nada.... Not sure the keyboard was alive and working. Mouse cursor would display and move.
So, cycled power, everything looks normal when booted back up.
What's going on??