Please fix this issue anyone, my desktop restarts everytime

While i do any little intensive tasks like play hd movie, youtube videos at 480p etc.. even while playing cs1.6 sometimes, it get restarts.

Specs :
ASROCK G41M-VS3-R2.0 Motherboard
Intel Dual Core E5400 @2.70GHz
PSU 450w

Ok this is not THE END.
My PC doesnt turn on after boot screen if i dont set bios settings manually..!!
What does it mean?
Well its mean, if i dont change some settings in bios, it doesn't turn on!
Settings that i changed is :
Overclock mode - Manual
CPU Frequency (mhz) - 90 (minimum is - 90 and default is 200, mean, underclocked to minimum frequency!)

DRAM Voltage - from Auto to '1.65v'
NB Voltage - from auto to '1.10v'
Vtt Voltage - from auto to '1.10v'
GTLRef Voltage - from auto to '0.63 x vtt'
[NOTE : ALL 4 voltage settings are underclocked]
Here is the status of my HW Monitor tab in bios
CPU Temp : 55C
Motherboard Temp : 40C

CPU Fan speed : N/A
Chasis Fan speed - N/A

Vcore : 1.336v
+3.30v : 2.960v
+5.00v : 5.064v
+12.00v : 12.144v

I guess my cpu fan is the responsible for this, i had broke my cpu fan (intel push pins style) and i changed with a cpu fan from my older mobo which is cooler master and it uses compound to set down. So now i think, it is the prob of cpu fan most probably. As far as i know, psu wont be the prob this case as the power is working without a prob.

so whats the main issue? how to solve it? please help me out some one Sad please!

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Clarification Request
This looks like an older PC

The CPU is from 2009 so this may be a very old PC. If you suspect the heatsink and you should if the BIOS temp is 55C when the machine is doing nothing, that's what you should tackle first.

The CPU Fan speed N/A may be telling us it's the wrong cooler as well.

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Clarification Request
Cpu fan

When you say cpu fan are you talking about the fan only or the heatsink and fan?

If it's the fan only.
Verify the heatsink is attached proper to the mobo.
Verify the heatsink is clean.
Verify that you did not mount the fan upside down.
Air flow should be into the heatsink.

If it's the heatsink and fan assembly.
Did you use new thermal paste?

As for the bios replace the bios battery.

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May be your CPU fan

Yes may be the CPU fan is the problem . As the processor is quite old, due to thermal throttling the computer is restarting it self to avoid damage.
Or if you are confident with that your PSU is not having any problem then it may be your HDD.

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Reseat cpu cooler with fresh thermal paste

Remove the cpu cooler (the heatsink and fan). Use a very soft clean cloth to remove most of the old thermal paste from the cpu and the heatsink. Then use a cotton swab dipped in isopropyl alcohol to clean off any remaining thermal paste residue. Gently polish both surfaces with a clean soft cloth. Apply a dab (about the size of a fat grain of rice) of Arctic Silver 5 thermal paste. Reseat the heatsink/fan and MAKE SURE it's securely in place. For example, when using the Intel push-pin style fitting, if one push-pin is not securely down the cpu will easily overheat and shutdown/restart to avoid damage. As others have said, this is the most likely cause of your problem. Also, be sure to blow all the dust from the heatsink fins and fan with compressed air. Dust buildup in the heatsink fins also causes overheating/shutdown.

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