There is NO single cause or cure. There is also the NEW plague of Spyware. If the machine has spyware, then it can have this issue.
Very frequently (every half hour or so!) my Dell Inspiron 4000 freezes totally and absolutely. Cntr/Alt/Delete does nothing except if I do it a few times I get blank screen and then a blue box saying something to the effect that the computer is waiting for the Close Program dialog ... and that I can press any key to return to Windows (nothing happens) or press cntr/alt/delete to restart computer (nothing happens). I have to press power button for ages till the computer turns off and then start up again. Any ideas?