computer freeze

Every OTHER time my computer boots up, it freezes. The mouse does not work while programs are being loaded during startup. Any thoughts? I am running XP pro and occasionally the system shuts down, with a blue screen which reads the computer is shutting down, unstable. I have 425 watts power supply, amd 3500 939 chip, MSI KN8 Neo2 motherboard, 1 gb Kingston value RAM, ATI 9600 XT all-in-wonder graphics card.