Computer restarts whenever I shut down

[#1]

My computer has recently developed a flaw.
Whenever I shut down in the usual way (Turn off, not restart) Windows closes in the usual way then after a few seconds the computer reboots.
I dont know if windows is setting it to reboot or if its a hardware setup. Either way I cant find anything in the settings to fix it.
Could anyone please advise
Thanks

[#2]

It will probably be a slightly flaky bit of hardware (or a driver that's been updated) and that won't turn off properly as a result.
You could try and figure it out by taking things out and/or disabling them but it could take ages. And if it turns out to be a motherboard item you are stuck.
I've had many similar symptoms and just learned to deal with them as fixing just takes too long.
Examples include:
1. Reboot on power off (as you have)
2. System locks just before it should power off
3. System halts with error message just before it should power off
Sometimes I've even worked out what the "guilty party" is but the minimal effort to hold the power button for a few seconds to kill it was too easy in comparison to finding & fixing the problem.

[#3]

Hi
Reboot on power off is the only symtom I've had so far
I hope it's not the motherboard because thats brand new

[#4]

This can be a pain MS made it so you can track what loads at startup but not on shutdown.
In control panel \system\advanced\startup and recovery untick automatic restart on error.
As windows finds an error on shut down it restarts so this may stop it.
Then check the event log to see if there is anything there.
When you shut down close all programs first then try exit on anything in the system tray see if that works if it does its a matter of exiting one at a time until you find the problem. This would include things like AV, bluetooth, printers, email
Any new software or hardware recently? 
You dont have windows search installed do you as that is a known problem.