why does computer crash most days

I run 2 Samsung Syncmaster monitors 225BW/P2270 1680x1050 & 1920x1080.

PC is Hewlett Packard
AMD Phenom?9750 Quad core Processor 2.4Ghz
Ram 4GB 32bit system running windows vista home premium
ATI Radeon HD3400 series

Ever since i purchased, probably been over a year now, PC crashes usually once per day, maybe 80% of the time, other 20% crash free days. There's no particular time or pattern as to when it freezes.

It's under warranty at Harvey Norman but for their techs to have a look could cost me $$$ in lost time.

It always restarts ok and will run without any problems for the remainder of the day once it has crashed.

Does anyone have any tech knowledge that can give me clues as to why this happens?

thanks in advance