My computer turns of suddenly after being on for certain amount of time.

[Clrrr]

So my computer would turn on not that long ago for more than 30 seconds. The fan had a problem for the fan on the motherboard, so I bought a new one and replaced it.
 Now my computer turns on for about an hour and then turns off suddenly. The first time it happend I though it might have been bad contact and turned it on again, but the second time it only stayed on for about half of the time. I stopped trying after that for the day. I turned it on the day after and it stayed on for about an hour, hour and a halfish. I havn't turned it on since. The fan on the computer computer is working fine, but there is only one fan. It used to work fine like this with the just one fan before it broke, but now it isn't.  Is it overheating, do I need a new fan, or is it something else. Any help would be appreciated. Thanks.

[Ganesh]

its not a fan problem. it should be smps or mother board problem. first check your smps that it giving useful power to cpu. if it is ok, then please check your motherboard.

[Clrrr]

Thank you Ganesh. How do I determine whether itīs a motherboard or power supply problem and what the problem is?

[Geek-9pm]

There is a free program to report temperatures of the motherboard and other things.
http://www.almico.com/speedfan.php
Wait!  Your AV may block the above link. Below is a safe link fro speedfab abnd similar programs.
http://alternativeto.net/software/speedfan/

[Clrrr]

I figured out what it was. The computer was overheating and it was the fan, actually the thermal past the guy who put it in didnīt do it right it wasnīt covering the whole back part of it on the heatsink or whatever and he didnīt clean off the dried up thermal paste on the backside of it from before. I cleaned it off put new thermal paste on and now the cpu doesnīt get over 46 degees c with the gpu only going to 65 c.