While there are a myriad of possibilities, three things come to mind; A) hardware error. Esp memory. Rare, but that just makes it harder to spot. B) ram. If y

While there are a myriad of possibilities, three things come to mind; A) hardware error. Esp memory. Rare, but that just makes it harder to spot. B) ram. If your open programs exceed physical ram then virtual mem kicks in, and it is slow. If the working-set ram is below physical ram the machine thrashes making it more or less unusable C) cpu. Programs can gobble cpu generally slowing the machine down. This is less noticeable these days with multiple cores. I don't think this is the cause of your problem but inspecting task manager can be helpful. D) drivers running in ring 0.these can crash a machine, but bad drivers these days for out the box hardware is very uncommon.