Format your pc. If you still have originl configuration as set by the factory, you can bring it back to its original form. goto hp site and find out the steps needed to restore your system. Pls note, you will loose your data using this option so might as well do a backup before you perform this activity.

most probably, your system is infected with a bug which is causing alot of processes to run which is causing vista to take preventive measure and shutting it down.