WHAT WOULD CAUSE MY COMPUTOR TO SHUT DOWN AFTER I START IT ABOUT 5MIN

You've provided too few info.

It's impossible to guess the reason w/o additional facts.

Is it switch off immediately or it peforms correct Windows shut down procedure? Or may be it just reboots?

First leads to a hardware failure, most probably power supply unit or motherboard (less probably). Try to just replace PSU at first (get one for test from your neighbor or friend, you just need to make connections, no need to install PSU to the case).

Second leads to a software problem, such as virus activity or driver failure. Try to explore events in Windows Event viewer. You may need to call pro to find the failure.

Third may lead to both hardware or software problem. Very wide variety of problems. You'd provided no more symptoms. Most probably profy required at the place. You've provided too few info.
It's impossible to guess the reason w/o additional facts.
Is it switch off immediately or it peforms correct Windows shut down procedure? Or may be it just reboots?
First leads to a hardware failure, most probably power supply unit or motherboard (less probably). Try to just replace PSU at first (get one for test from your neighbor or friend, you just need to make connections, no need to install PSU to the case).
Second leads to a software problem, such as virus activity or driver failure. Try to explore events in Windows Event viewer. You may need to call pro to find the failure.
Third may lead to both hardware or software problem. Very wide variety of problems. You'd provided no more symptoms. Most probably profy required at the place.