After login PC reboot or logoff automatically. What could be tha cause? Need help

Sometimes PC reboot because of a bad fan inside the computer, it could be the fan of the processor or the fan of the box, check at the back if the fan is running and check the fan of the power supply too.

Check your power supply

This can also be caused by a corrupted Registry. Try booting into Safe Mode, try using Last Known Good Configuration (pressing the F8 key at startup). If all fails I would put the hard drive in another PC to save the data and reinstall Windows.

I my case memory caused this failure

Check your power supply if the problem persist write click my computer go to advance tab
Click startup and recovery click on settings and untick automatic restart apply and click OK

Are there any sounds before the PC boots up?

Change the operating system

Its your hard drive over heating I had the same prob make sure all the case fans are running

Cheek your memory unit, and power supply if they are working well, restart you system and press f8 when coming up choose boot with safe mode.
If the system boot with safe mode compilily you may have prop with some of your hardware