I own a compaq presario desktop the system gets hanged after I use it for more than an hour and the only way to shut down is to switch off by pressing the power switch for some time.Other methods (Cntr+Alt+Delete) has no effect.

Sounds like maybe your mother board or processor is over heating. Take a look inside your computer and see if both fans are running. If they are... it may be a hard drive problem. I don't know the exact specs for how hot your processor should get so I can't tell you what it should be. I'm going to say it's most likely a heat problem though. It may be an easy fix just to replace the bad fan is there is one.