Not sure why,mine is doing kind of the same thing,i have a 531s,and McAfee virus,anyway when in screen saver,after about 1 to 30 min it will freeze,and have to kill power to restart,and says windows did not shut down properly,etc.,i have had it two weeks,still cannot find why,have wrote dell,e-mail no-reply ,went to microsoft site downloaded everything,that sound like could help,also downloaded all recommended,downloads on dell site,(did not help)..am strongly thinking of dumping this vista,and loading xp, used it for years(no-problem)  good luck,,