PC shuts down whilst gaming

Hi there,
My PC has just started shutting down with no warning whatsoever whilst playing games. I only play a couple but it happened the other day with Sims 3 and has happened a few times on Portal 2. In the event logs all I can see is an Error from afterwards that says "The previous system shutdown at 15:13:13 on ‎24/‎04/‎2011 was unexpected."
I can't see much dust around in the PC so I don't think it's that. Also fairly sure I don't have any viruses or the like. Was monitoring my GPU's temperature whilst playing Portal 2 and the MemIO was peaking at about 120 degrees C, and DispIO and Shader were at about 110 degrees C. They were all fluctuating between 99 and 110+ degress C. Don't know if that's high for a GPU? GPU fan is running at about 5250 RPM.
I'm running Windows 7 Ultimate x64 with a quad core AMD phenom x4 black edition 9850 (2500 MHz), 8GB ram, ATI Radeon HD 4850 (512mb) GDDR3, 1x 160gb HDD (only about 4% space left), 2x 1tb HDD, 1x 2tb HDD. PSU is a Tagan 2 Force II 600w and attached is a picture of the sticker on the side. Hope I've given enough info :-D

 

If this is your boot drive, clean it up so that it has around 33% free space.

Are you overclocking anything?

What make/model motherboard is it?

 

Yes it is my boot drive, I'll clean that up then. Motherboard is an Asus M3A-H/HDMI. no overclocking.

 

OK, 160gb boot drive now at 40% free space

 

Ok, now give it a defrag and it might play better

 

doing that now, will let you know if it happens again! p.s. a second screen wouldn't make this happen would it?

 

Mmm, a second screen would load the gfx card more, maybe increase the heat a little. If something's already close to the limit, it only takes a very small % increase to push it over the edge.

 

basically i thought it might have been the second screen so i disabled it with ultramon and it seemed to work fine for the night, then it shut down again even when it was disabled. does this mean i should unplug it from the graphics card each time i want to play? that'd suck lol.

 

Personally, I try to use the built-in Windows tools whenever possible - less chance (I hope) of incompatibility creeping in and confusing things. I'd disable it from Windows Device Manager.