Windows 7 - PC turns off suddenly when booting

Sometimes when I power-up my pc everything goes on (fans, HDDs, MOBO..) 2-3 sec later it shuts (won't even get to the bios boot screen) then few seconds later it will turn itself on and stay on (log into windows..)

As I stated before it happens sometimes, any suggestion?

The first thing I think of with that symptom is your power supply.

Now normally I would suggest that your power supply could be failing, and that might still be the case, but when I saw that you have only a 450W supply and the components you are trying to run off of it that raises the possibility that the power supply might be just too small for your system. And the older that supply is the more of an issue the size becomes.

Go here: eXtreme Power Supply Calculator and fill out the form to calculate the size your supply should be for your system. The most important box to fill out accurately is the "Capacitor Aging" at the end. That is where the age factor gets worked in.

TVeblen may very well be correct. The minimum PSU recommended by EVGA for your card is 450W with 22A which is what you have. Time will lower that somewhat as PSU's will degrade with age. It could also be a bios setting. Try resetting bios to optimized defaults and see if that helps you problem.

IMHO never run a minimum power supply. You will work them to hard which creates a lot of heat. For that system I would recommend at least 650w quality power supply. Antec and Corsair come to mind.

The calculator noted 363W as a minimum and 413W Recommended (Cap Age 10%) even though my PSU is 2 weeks old

oh and essenbe, a BIOS Setting? mmm I'll try that

That puts your PSU running at almost 100% all the time. Most PSU's are not capable of doing that. What I try to do is run my PSU at 60-70%. That makes sure I have plenty of power and keeps my PSU cool and will make it last longer. I always recommend Corsair, Antec and Seasonic PSU's.

I have found that those calculators just get you in the ball park. Yes Seasonic is another quality power supply.

So i should upgrade my PSU then, you guys don't recommend TT as far I see?

Since your power supply is brand new, and rated above the recommended wattage, then that means that a too-small supply may not be the immediate cause of your problem.

Keep in mind that does not change anything the other posters are saying. I agree with them completely - a borderline PS is not a good idea. I'm just saying that, for now, that may be a separate issue.

The issue for now may be that the power supply is defective - meaning it is intermittently not putting out all of it's rated power.
But it could be other things too.

A lot happens when you push the button on your computer and before you see that first BIOS screen. Your issue could be due to anything that happens during that 2 seconds of time that could interrupt the process to begin the BIOS startup.

When you press that power button it simply closes a circuit between the case button and the PWR SW header on the motherboard (2 wires - they simply contact together).
So the first point of error is that switch. But your switch seems to work fine because you get to step 2:

When that PWR SW circuit is closed it basically sends a signal along a "logic circuit" on the motherboard to the Power Supply telling it to switch from a low-power "standby" mode to the "full power mode".
So the second point of error is that logic circuit. Any defect and the power supply will not switch on.

When the power supply is switched on that activates the BIOS which then performs a POST routine (Power On Self Test). The routine checks the CPU, the RAM, and the System Bus, (and more) for connectivity and compares what it finds against it's stored "memory" of what should be there. If all checks out you will normally get a single BEEP from the BIOS - the POST Beep. Unfortunately, many modern systems have eliminated the POST Beep - yours may be one of them.
That's the third point of error. If the POST does not complete the boot process will stop. This indicates that there is either a major component failure (including a lack of, or disruption of, power) or there is a discrepancy between what the BIOS finds and what it "remembers" should be there.
This is why Essenbe recommends resetting or restoring the BIOS defaults. That process will clear up (refresh) any discrepancies, if this is the case.

Once the POST is complete the BIOS begins the remainder of the BOOT process: discovering installed components, drives, and devices of the system.

So it is very important for you to determine where in the start up process your system fails. And being an intermittent problem makes that more difficult.

If the system shuts down before you get that POST Beep, or before anything shows up on the monitor, then that indicates you have a problem in the power category. Could be your case switch, could be the connections on the motherboard (double check 'em!), could be the power supply itself.

To test the Power Switch on your case is pretty easy. Just pull the connector from the power switch on the motherboard (the PWR-SW header) and then use a small flat blade screwdriver to touch the 2 pins together momentarily. This should turn your system on. If the system displays the same problem when you switch it on this way then that would eliminate the case switch as a cause.

Let us know what you find.

PSU's are a matter of personal preference. But, I believe that looking at the models with the best user feedback is a wise way of choosing one. Your psu may not be causing your problem but is suspect and is running at close to it's capabilities. At this point you have no room for any upgrades and your usage will shorten the life of the PSU. So, yes I would recommend a higher wattage PSU. You will be doing yourself a favor and allow you room to upgrade in the future. The brands I mentioned are the ones that have the fewest problems and better quality (my opinion). Read some of the viewer feedback on PSU's at Newegg, even if you don't buy there. And remember, the PSU is not a component to save money with, it is a very important part of your computer. If the PSU goes out, it can take many of the other components with it. The better quality ones are less likely to do that. Just my opinion.