Run away HD /Machine freeze up

Over the pase few weeks, my Inspiron 454 simply freezes up; i.e. mouse will not work, trl+Alt+Del does not envoke task manager. I have to press the on/off to kill the machine and restart.

Any Ideas why?

Machine is protected by  Zome Alarm Firewall and AVG Anti-virus software.

In addition machine is acaned bt SpyBot Search and Destroy daily along with a virus scan done twice daily.