LOCK UP - FREEZE problem

Okay, I'm using this old dinosaur computer... the same one with the (now resolved) Picasa (non) issue.

If I don't do anything within a 15 minute period... say I walk away to go potty, have a smoke, etc., my whole system freezes... locks up. Control-Alt-Delete does nothing. The only way to get the computer to do ANYTHING is to use the "power switch" to turn the computer off.

When I turn the computer back on, it goes through it's abnormal shut down warnings, fixes, etc., then it's fine again.

The only thing I notice prior to doing the hard boot is a 1/4" strip of fine vertical stripes that runs all the way across the top of the screen. In case the color matters, those little fine stripes are bright green.

What say you?

~Vickie