Need some better details here, exactly what happens when you turn on the computer, it just cycles from on to off over and over? How far does it load before it reboots? If it tries to get to Windows then crashes, try re-seating the RAM, test with different RAM or one stick at a time, run a disk check utility and see if the hard drive is bad. If it crashes at or before the BIOS screen, it may be bad RAM, battery, motherboard, something with the hardware but less likely the hard drive.