Hi Guns and welcome to the forum.

We could do with a little bit more background detail on your machine specs to aid us here.

When you say 'Shuts down' do you literally mean everything shuts down or just you loose the graphics?

when the computer starts keep pressing F8 it should go to the OS choices menu. Don't boot into safe mode but look for anything that says 'DISABLE AUTOMATIC RESTART AFTER SYSTEM FAILIURE' . You want to disable the automatic restart feature. press enter to start normally. See what the error is if you can and write down and post on here.

It could be a bad PSU causing the problems but firstly I'd look at it being the RAM or the graphics card.

If possible take out the RAM and the graphics card and see if you can encourage it to fire up from there. If you can get it to do this then add one RAM stick at a time followed by the graphics card until you get the system failiure.

When I say 'shuts down' I mean everything shuts down, the whole system reboots. Also I cannot find a 'DISABLE AUTOMATIC RESTART AFTER SYSTEM FAILIURE' in the f8 options menu

The option should be down at the bottom of the screen but here is another option for you to try.

In the F8 options menu you have the choice to boot windows into 'SAFE MODE' Select this option and allow the system to boot into Safe Mode. If it boots successfully then the problem lays within some driver. SAFE MODE loads the OS without things such as the graphics drivers which is why everything will appear huge.
It is common for a driver problem to cause a PC to restart.
While in safe mode enter the command prompt and tell the machine to do a check disc upon restart
chkdsk/r

then
shutdown -r

this restarts the machine and upon restarting will force a 5 step check disk. Be patient this will take some time.

The option isn't at the bottom of the screen, and the computer will not boot up into safe mode

If the PC won't even boot into safe mode then it looks like you might have a hardware problem. Reduce the system right back down to barebones IE - PSU, Motherboard, 1 stick of memory, 1 hard drive. Remove all add in cards and other devices. try to boot, let me know what happens.

Ok so I removed all the addons. All that is left is the PSU motherboard 1 stick of memory a hardrive and the powersupply. It still will not boot up.

I just tried a different power supply and the problem still persists

It looks like in this case then that you have a problem laying within the prcessor or the motherboard then. Take out the processor assembley and ensure the processor is fimly seated. If no go then i'd buy a new motherboard.

Mine does that......... have two computers and they are both doing that........ starts, goes to Windows front page and then switches itself off and then starts again and keeps going like that in a loop...... but I can start it in Safe Mode and even network in Safe Mode.............
Will do a chkdsk/r and see what happens.......
Thanks