Hi can any1 help me my computer just keeps on restarting, its really annoying me.?

iv rebooted the computer up now, and still, after a little while since its been on, it just freezes for a couple of seconds, then restarts, theres also a blue screen that flashes b4 it restarts. i dnt know whether theres a problem with the power supply or what, but iv had it for 2 years the computer, and its just started 2 misbehave now...any help will b gr8 ty