your motherboard manufacturer should have a utility on their website for monitoring the the temps. its usually in the same place you find the drivers for your motherboard.
for video temps the NVIDIA Monitor temperature measurement utility, which is part of the nTune program can handle that.
getignited114 is most likely right, your overheating and my money is on the cpu or northbridge since it restarts rather than hangs. the gpu most of the time hangs rathers than restarts.
the easiest way to find the problem is to get your system running what makes it crash wait a few minutes then open the case and feel the heatsinks inside your case. (keep the case side panel on when its heating up it being open might make it run cooler) the one that burns you the worst is likely the problem. get more air to it.
is anything super hot? sometimes a chip is just made wrong and lets voltage jump across transistors, its not always heat to blame. ive had to underclock in the past to fix this issue. but if you think an area is getting bad air flow or is just overheating despite good air flow. do what makes sence to fix it. my biggest tips for air flow are.
-dont put your computer in a corner. it makes the hot air leaving your case go right back in. under desks is the worst.
-make the overall airflow in your case go from front to back. fans up front blow into the case and the ones in the back blow air out of the case.
-if you have a side duct for your processor use it. chuck a fan in there and let your cpu get nice and cool room temp air.
fans only use 5-20 watts just ignore their existance when calculating total wattage unless you have a lot.
I would say play the game again, not for extensive periods of time, and keep an eye of the temperatures using simple software such as HWMonitor to see what the temperatures are prior to shutdown (if it comes to that). Typically, temperatures will rise in gaming environments even if you are idle in them, so leaving the game on for a short while will generate results so long as you have a game running.
so should i put another fan?
will it increase the load on the psu?
the first thing you need to do is make sure your graphics drivers are completely up to date.
the second thing you need to do is follow the recommendations already posted. download HWMonitor or Real Temp and check your temp under load. you'll do this by opening up your temp monitoring software, and then playing assasin's creed for a little while. (how long just kind of depends, but 1/2 hour should be more than enough) when you close the game, you'll have max values for your cpu and gpu temps recorded in your monitoring software.
post these numbers in this thread, and then people will be able to be more specific with their help. for now, there isn't enough information to determine what you need to do to fix your issue.
i found the problem. my dust filters infront of air intake cooling fan were full of dust! after i cleaned the dust filter the pc didnt restart. i played about 12 hours straight for the past two days and the pc didnt restart. thankyou for your help people.