my PC just got its motherboard replaced (it was under warranty so It was replaced) and when I fitted it back it did not work in the sense that after a few hours my computer just um... Stopped just nothing suddenly everything stops and no error report or anything I use windows 8 and used it for a month without problems but then the mobo stopped working sent it for replacement and this has occurred for 4 times and I am still having the same problem.. so what should I do (I have checked every other component by replacing it(eg PSU,RAM, CMOS battery, CPU Fan, etc). Any help or solutions please...... (I am thinking of going to court now it's been like 50 Days now..)

hmm. This happens to me when I run @ 12GB of DDR3 RAM due to my old age PSU (Spent all of my money to build my new rig. Forgot to buy a new PSU). If you're using an old PSU, replace it but I think the motherboard you got is faulty. Try getting a 160GB HDD or a cheap one. Install Windows 7 in it and do some test playing video games or leave it running for an hour or more.

It can be:

 

Faulty Motherboard

PSU couldn't generate enough power (4-pin ATX on a 8-pin ATX)

Windows acting up

I have got a 500 watt Psu and yes there is a problem with it, because the fan does not start by itself I have to manually start it but I don't think that's the problem because I replaced it temporairily with a brand new one only to find the same crashes....

(I am thinking of going to court now it's been like 50 Days now..)

What are you, American?

Just ring them up and say this one is broken too, get it replaced.

You're talking about production processes with massive error rates, it's quite common that parts are DOA, you just had the bad luck of 2 in a row..

yeah that sounds like a cooling/heat-sink problem.......

Check the heatsinks and make sure it's properly mounted. If everything is fine, replace the thermal paste.