E1505 System Hang - CPU 100% when we browse Internet & normal usage

This is happening since the time I bought the system late 2006.  System just chocks.  Nothing works  Not sure how to resolve the problem.  The only thing we do is just reboot the system and use it.  If it hangs just reboot and work.  I have reinstalled Windows lot of times, but the problem is still the same.  I also had 3 year extended warranty from Dell, but the tech support says to reinstall windows.  When I reinstall also it comes back.  Not sure what is the problem.  Can somebody suggest?  I have also run hardware diagnostics

Regards,

Giridhara