Why does my computer lock up or slow down after I leave it on for more than 7-8 hours in a stretch and use it again... it is either lagging or frozen. Its a 2.6 mhz P4 w/ 512 mb RAM. I use XP. When I click on any programs they don't open nor does the shut down or restart work. So I have to manually turn it off and on again. And then it works again. ???