The OS is ME (Hewlett Packard). Just about 3/4 of the times, I have to shut off the stupid thing via the power strip. I shut it off the proper way, but most of the time it will shut down "not quite" that is to say, the fan stays running (screen is off, power button lights off, etc.). It's been doing this almost since it was new so it's not an acute situation. It's not loaded with a million programs and there's at least 80% free disc space. I've had various firewall and virus programs over the years and it makes no difference. I know that the Millenium Edition is the worst operating system, but I've never heard of anybody else with this problem. Somebody on CyberTech said it was dust inside the case. Well, I cleaned it out (not much dust, really) but it made no difference.