All my games are crashing!

[OverThrownBaby]

I recently finished building my computer from the parts I received for Christmas. First day it seemed everything worked perfectly. No lag in any game and no sign of graphics problems. Now though after getting more games it seems that just about all my games are crashing.
Here are a list of the games that are crashing and how they crash.

Team Fortress 2: Randomly Exits from 20-60 minutes of play with the error hl2.exe has stopped working.
Gmod: Randomly Exits from 20-60 minutes of play with the error hl2.exe has stopped working.
CS:Source: Rarely Crashes, but it has after 2 hours before.
Arma 2 OA: Crashes Fast 3-20 minutes, ussually freezes up and my computer automatically restarts itself.
Crysis 2: Crashes Fast 5-10 minutes, Blue Screen.
Planetside 2: Only occastionally crashes after 30 minutes. Exits To Desktop.
GTA4: 30-60 minutes,Randomly Exits To Desktop

Please note that before my games ran perfectly, so I'm thinking it's not hardware.

My Build:
Processor: Intel i5 3570k http://www.newegg.com/Product/Product.aspx?Item=N82E16819116504&Tpk=3570k
MOBO: ASUS P8Z77-V LK LGA 1155 http://www.newegg.com/Product/Product.aspx?Item=N82E16813131837
RAM: G.SKILL Ripjaws X Series 8GB http://www.newegg.com/Product/Product.aspx?Item=N82E16820231455
GPU: EVGA Gtx 560ti (link might not be the exact one) http://www.newegg.com/Product/Product.aspx?Item=N82E16814130604
Power Supply: Cooler Maser Silent Pro 800W Gold http://www.newegg.com/Product/Product.aspx?Item=N82E16817171057
Hard Drive: Seagate Barracuda ST1000DM003 1TB http://www.newegg.com/Product/Product.aspx?Item=22-148-840&SortField=0&SummaryType=0&Pagesize=10&PurchaseMark=&SelectedRating=-1&VideoOnlyMark=False&VendorMark=&IsFeedbackTab=true&Page=2#scrollFullInfo
SSD: Samsung 120 gb If I find the link I'll put it here.


I have only really looked at the CPU for temperature problems. After playing games for about an hour and then crashing I quickly checked the temps and they were at a cool 31-35 Celsius.

Please Reply. I will attempt to work with whatever you can give me. I really need to solve this.

 

[EzioAs]

Update every drivers (chipset, sound, video) first. Update directx as well. If you're overclocking any parts (cpu, RAM, gpu), ran them at stock speed first.

Run prime95(blend) , furmark and memtest to see whether you get any errors or not. Monitor temps using open hardware monitor

 

[Sumukh_Bhagat]

Check your GPU Temps
Use MSI Afterburner.

Crashes usually happen when your Card gets very hot.

 

[bigshootr8]

+1 to sumuk and I had that card and it was one hot brick. Great card just ran super hot all the time.