1) Clean your PC - with Pressurized air (And with care)
Remember to give the Powersupply (PSU) a good "Blow" (Sorry)
2) Check your CPU FAN - is it rotating ok ?
3) Check your PSU FAN - is it running ok ?
4) Check your RAM - use Memtest 3,5 or related
5) Give your harddisk a thorough surface-check (chkdsk etc.)
6) Visually inspect your motherboard for bulging or leaking Caps
7) Try another high quality PSU - or at least a PSU at sufficient wattage
8) Dismantle your PSU (Or let somebody else do it - its not completely safe for unathorized personnel, etc.)
Inspect the Caps (Electrolytic capacitors) - see if they are bulging or leaking some yellowish / brownish stuff from top or bottom.
(Electrolytic Capacitors or "Caps" - are typically small cylindric boxes - looks like a little battery - app. 10-12 mm wide on motherboards - from 10-25 mm in a PSU.
A PSU have 10-20 of these, Motherboards have 20-60 placed all around. Usually there is a cluster or row of these just next to the CPU - and these are the most exposed to "Capacitor plague" (Google this).
If Bad Caps are the case (And thats my bet) - get qualified personnel to repair it, or buy a new PSU of good quality (All SeaSonics, PC Power & Cooling, following Corsair models : VX450, HX520/ HX620, TX650 - they are all of SeaSonic design - and in any way you look at it, superior the other Corsair Units (Channel Well design).
The question is : Which power supply do you have now - and how old is your machine ?
If Antec - replace, if some Cheap OEM Model - Replace, if Deer (Brand) - replace, etc.
Let me give you an example :
App. 3-4 years ago i had a PC go unstable - after 2-5 minutes of battlefield gaming - the PC just restarted without any obvious reason.
The PC had a Cheapo 550 watts PSU - I changed it with a quality 300 watt rebuild/recapped Fortron-Source spare from the shelf - and afterwards the machine could play battlefield for hours without problems - the thing that made it go unstable was of course the 550 watt crappy PSU - which in reality didnt deliver 550 watts of power - nor the ripple free and stable power needed to drive the newly mounted 6800GTS VGA card.
Many Power supplies are actually build with cheap chinese capacitors - they don´t last. If same bad quality Caps are used on the motherboard, it won´t stay alive for extended periods either (1-5 years depending on use) - So if none of the other qualified posts helped you solve your problem - Try googling for "Bad caps" - and you will see why i change soldertips on my soldering station every week......
One of the main reasons these cheap caps don´t last, is the lack of "Dustbusting" inside the PC - once the dust clouds up - it will shorten the lifespan of already crappy components - so its CRUCIAL that you clean your PC completely inside, at least every 3-6 months.
including your CPU cooler - NB cooler, PSU, VRM Circuit, PSU, etc.
PS : When you have a problem like this - it´s essential that you tell us what hardware you have - by knowing the exact PSU name or model for example, and the make and model of your motherboard, would help us solve your problem in a rush...