assembled pc

I bought one assembled pc with the following configuration :-pentium4 ,d850 intel motherboard ,20 gb hardisk ,256 ram & agp card with 32 ram memory .
Th problem it hangs with no noticeble reason & if i change vga card resolution to 800*600 true color 32 bit
the pc starts shuting down & restarting
besides that the pc usage goes to 100%
once & then goes back normal .
is there any pc utility which can diagnose an pc hardware .