So about a month ago or so I got a new gaming PC, Im really happy with it performance wise, but lately, it's been randomly rebooting itself without any aparent reason. Happened twice when i was playing Kotor, once when I was browsing on Firefox (clicked some link on GB actually and it crashed and rebooted) and once when I clicked my anti virus. So I have no idea what is causing it, or what to do? Maybe it's getting too hot? I have it pretty far from the wall (the back part) and both sides are ventilated. Can I actually check the core temperature somehow? Or maybe check something else? Any help would be apreciated.

Is it instantly shutting itself down or does it pause, blue screen, then restart?  

" Is it instantly shutting itself down or does it pause, blue screen, then restart?   "

In my experience 9 out of 10 times it is the memory. Incompatible memory will work temporarily but will randomly crash and restart. If you can't verify that your memory is compatible try getting one that you know is compatible and check it out. This kind of info can be found online. I will help if you need it.  

At this point in time it sounds like it could be any issue. Let's check temperatures first; download RealTemp and record the idle and load temperature of both your CPU and GPU. 

1) If anything is overclocked, revert everything back to stock settings.
2) Run a memory test, and make sure your memory has not somehow gone bad.
 
To check temps use can use Speedfan, RealTemp, CoreTemp, etc or whatever other software shipped with your board. Any of those will do.
Usually, if it's a temp issue, you'd get shutdown and not restarts. Same for power problems. I'd be looking at the memory first thing.

" 1) If anything is overclocked, revert everything back to stock settings. 2) Run a memory test, and make sure your memory has not somehow gone bad.  To check temps use can use Speedfan, RealTemp, CoreTemp, etc or whatever other software shipped with your board. Any of those will do. Usually, if it's a temp issue, you'd get shutdown and not restarts. Same for power problems. I'd be looking at the memory first thing. "

" @SlasherMan said:

" 1) If anything is overclocked, revert everything back to stock settings. 2) Run a memory test, and make sure your memory has not somehow gone bad.  To check temps use can use Speedfan, RealTemp, CoreTemp, etc or whatever other software shipped with your board. Any of those will do. Usually, if it's a temp issue, you'd get shutdown and not restarts. Same for power problems. I'd be looking at the memory first thing. "

I didnt overclock anything. how do I run a memory test? yes, im a nub >_> "

 100% run speedfan and realtemp while you play a game (or while running Prime95 as mentioned below), make sure nothing is getting too hot. 
 
Inside your computer case, check that the rear fan is blowing out of the case and not in. And that the front case fans blow in, not out.  
 
Memtest86 is fantastic take peoples advice and use it! Another great program is Prime95.   I would get both for future burn in sessions.  For Prime95, run the "memory blend test".   That will find errors pretty quick.   [but memtest86 is great so either one]. 
 
Then check all other drivers.   Did you do a clean install of windows 7?  
 
 
nevermind didn't notice the guy above already recommended Prime95, so I got nothing new to add. 

That sucks.
 
Did you install the special motherboard drivers after
installing your OS?  Check your Devices under System
& see if one of the drivers hasn't been installed.
 
If you have warranty on your PC or its parts, now's
the time to go take it back.  Typically I don't & what I
will do is make a backup of my system, then reinstall
the OS clean & see if it still has the same problem or
not.  If it doesn't, then I use the new install. Otherwise,
I put my backup back online & try again.
 
Also, I noticed random reboots may also mean
something wrong with the hard drive.  Run disk
scan.

Instant reboots are weird and harder to isolate. In my experience a memory issue will usually trigger a Windows blue screen warning before rebooting, or more obvious symptoms before the OS can even boot.

I agree with all of the suggestions so far. Run SpeedFan and check your memory with Memtest86. I've used both of these to diagnose similar problems before.
 
@Rhaknar said:

" so i fixed the problem about a week ago (at least it hasnt happened since) doing something a friend of mine (that put the rig together actually) told me to do. Not sure if I know how to explain it, but I had to change something in the bios to make it so one of the cores in the quad-core doesnt go "to sleep" when not needed, if that makes sense. Hasnt happened since so I'm guessing that was it. I'm sure someone can explain it better than me, if I'm making any sense whatsoever :p "

maybe you are lucky, that it's not happening now, because you said it didn't happen for a month after the original build. The problem may still exist.
 
You say the system reboots as if someone hit the reset button.
 
Have you checked in the control panel, in the advanced settings of System?
 
start>control panel>system> advanced settings? (I don't know your OS). 
 
There is a button in the "Start up and recovery" area that is called "Automatically Restart" . 
 
It is usually checked by default. If you uncheck this box, then you will be shown the blue screen of death so you can identify the problem better. However there must be an account of this crash in the logs as well...

Open the 'Event Viewer' that is located in the 'Administrative Tools' folder inside of the control panel. Look for the Windows logs and look at the System logs. Scroll for the approximate date/time the crash occurred.

Could be: RAM, PSU, virus. Pick one.