basically i ordered a custom pc they installed windows 7 and when i got it i installed all the updates and drivers yet if i maxiumised firefox it crashs or load a game or click on windows system rating :/ 

This is my new pc:

 

CPU: AMD FX 8150 Black Edition

 

CPU Cooler: Corsair H80 Water Cooler

 

Operating System: Microsoft® Windows 7 Home Premium 64-bit

 

Motherboard: Asus Sabertooth 990FX

 

RAM:  16GB Corsair DDR3 1600mhz Vengeance (2x8GB)

Hard Drive:  Corsair 120GB Force3 SSD S-ATAIII 6.0Gb/s
Additional Storage:  1TB S-ATAIII 6.0Gb/s
Optical Drive:  22x DVD±RW DL S-ATA
Graphics card: AMD Radeon HD 7970 3GB
Sound card: Asus Xonar DGX 5.1 PCI-E
Internet:  Wireless 802.11N 300Mbps MIMO PCI card
Case:  Zalman Z9 Plus
PSU:  700W Xigmatek

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Send it back and get your money back because they're using a backstreet crap PSU to power that setup, so it sounds like any time there is any real graphics pull, which it is under Firefox (hardware acceleration) and WEI (Direc3D tests), it's crashing due to lack of power.

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Crosshair IV Formula, Phenom II x4 965 w/ Corsair H60, 8 GiB OCZ Platinum DDR3 1600, XFX HD 7970 Ghz, 512GB Vertex 4, 256GB Vector, 240GB Agility 3, Creative X-Fi Titanium w/ Creative Gigaworks S750, SeaSonic X750, HP ZR2440w, Win 7 Ultimate x64

cheers dude im going to ring them up at 10am and see what they say because its annoying and spent soo much money on it :/ what PSU should i go for if i want to replace it?

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Xigmatek use HEC to manufacturer their PSU's... at least most of them. And they are... garbage.. >.>

Corsair, Seasonic, Enermax, XFX, Sapphire, PC power and cooling , Zippy are all PSU manufacturers who go above and beyond in quality, performance, and warranty.

I'd go with a TX650 from corsair, or if you plan on crossfiring in the future a TX or HX 850. Depending on your budget. But also make sure the card they sent you isn't bad as well. 

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I5 3570K@4~5.0ghz: Gigabyte GA-Z77X-UD3H:Corsair Hydro H70w/Delta 240CFM Fan: 16GB Corsair Vengeance DDR-3 1600: 2 XFX Radeon 7950 @ 1150/1575 : Corsair AX 860 PSU: x2 WD Black 1TB HDD's: 90GB OCZ Agility 3:CL X-FI:Corsair 500R Arctic White Edition:3 22inch Asus monitors Eyefinity @ 5294x 1050(bezel comp) : Alienware Tact-X Keyboard: Naga Razer gaming mouse:Corsair Vengance 1500 Headset .