Windows XP – Unexpectedly Hibernates

If I leave my HP Pavillon s7545a desktop computer idel for some time, around 20-30 minutes, the computer displays suddenly turns black. I assume, this meant that it goes on hibernate mode. So, disabled hibernate mode and set all the power settings to Never and yet, it still hibernates. Worse is that when I move the mouse or touch the keyboard, it takes about 30 minutes to an hour before the whole system is back on? What is wrong with my PC? Is it some hibernate mode setting I don’t know about? Has it got anything to do with Norton firewall, internet security and antivirus setting? Or is it hardware related?