System crashing while fans freak out

[Narinerd]

A few months ago I built my first computer, and it worked great until a few weeks ago when it started crashing. When it crashes, my screen, mouse, and keyboard lose power; whatever game music that was playing keeps playing on a loop; and all the fans start going really fast and make a ton of noise.

A few more details I've noticed about the crashes: As far as I can tell, they always start happening while I'm playing League of Legends and using Vent. I always have to force the computer to shut down by holding down the power button, and after the initial crash, it crashes within the first couple of minutes from when I start it back up, if not the second I turn it on. It seems to be okay if I leave it alone for day or two, but it keeps happening.

I'm not too experience with this stuff, and I do not know where to even begin troubleshooting. I've had a few people tell me it sounds like hardware issues, but I never get any solid answers or advice. Sorry if this was too much to read, but I really appreciate any advice you might have.

Here's my build:

CPU: AMD Phenom II X4 955
GPU: SAPPHIRE Radeon HD 6870
RAM: Kingston HyperX Blu 4GB
Motherboard: ASRock 990FX Extreme3
Power Supply: OCZ ZX Series 850W
SSD: OCZ Vertex 3 90G
Case: Rosewill CHALLENGER
DVD Burner: ASUS 24X DVD Burner
CPU Fan: COOLER MASTER Hyper 212 Plus

 

[Curtis772]

Install HWMonitor (http://www.cpuid.com/softwares/hwmonitor.html) and monitor the temps to see if you see anything unusually high.

I'm going to guess that your rig is overheating.

 

[Narinerd]

I downloaded that and the max temperatures I've gotten are 47 C for my CPU and 68 C for my GPU. But the GPU usually hangs around 50.

My SSD only has 90 Gigs of space, and most of that is filled. I have about 18.2 Gigs free. One of my friends heard that Hard Drives are supposed to have about half their space free to work properly. Sounds strange to me, but could this be the cause of my problems? Should I just buy another Hard Drive?