My computer often crash into a white screen. What is likely wrong?

start out with checking that the cpu/gpu not over heating. download hardware monitor and check the temps. download and run memtest86 to see if you have a bad ram stick. if you built the system yourself check that the ram is on the mb qal list or the memory vendor tested your ram in your mb. (check to see if there any bios fixes for your mb). if the ram isfine and the cpu temps are fine post up the size and make of your power supply it may be under powered or a cheap one that not up to the load.
with pc also make sure your video driver and mb chipset drivers are up to date as well as flash..java and web browser and any ap your using.