Screen goes black. Bought new computer and installed video card and still screen is

A friend of mine started having a problem with his old dual core computer where the computer seemed like it was booting up but the screen stayed black. This started when he got back from a weeks vacation and found out he lost power in his house. The power was back on when he got home but he could tell with clock being wrong, and neighbors telling him about the outage.He leaves his computer on all the time and when he got home he saw that it was off. When he booted up the only thing he could get was a screen with the XFX logo from the maker of his Geforce GTX 460 video card. Sometimes he wouldn't even get that, just a black screen.
He also connects his usb keyboard in the front and hit the connect to his computer and bent up the keyboard usb connector. I got him another keyboard and replaced the PSU thinking maybe with the power outage the PSU was effected. The computer seemed to boot up and work. Problem is it only worked for about a day (he played video games all day). That night he started with the problem again of black screen.
I thought maybe it was the motherboard then ( the system is an older dual core from 2007), so he purchased the following:

Gateway ModelX4870-UB318
SKU: 6835451
Windows 8 64-bit
Technical details: 3rd Gen Intel® Core™ i5 processor; 8GB memory; 1TB hard drive
Special features: Built-in wireless networking; onboard video with HDMI output
300 watt PSU

Being a gamer he needed more then the onboard video and I didn't think there was anything wrong with the GTX 460 geforce video card from his older system that started having problems, so I gave him a Coolmax 450watt PSU and installed the geforce gtx video card. The new computer booted up fine and I then downloaded the driver updates for the video card and started to install it. Shortly after I started the install the screen went black and after trying to restart the screen it still stayed black.
Anyone have any idea why?
I'm not sure anymore what was wrong with his old dual core computer, whether it was the motherboard, PSU, or video card.
Is it possible the 450watt PSU isn't powerfull enough (even though it did originally booted okay).
I took his video card and tried it in my computer with no issues. I left it on for a couple of hours and no problems.
How likey is it that his new computer has issues and should he take it back?

 

Re: Screen goes black. Bought new computer and installed video card and still screen

If a new computer begins to exhibit behavior from the old one, I'd remove the commonalities between the both.

You really should have tested the new computer with the onboard GPU and PSU first. Now you need to remove those two and put it back to the original config and test it.

It is also possible that the PSU isn't enough for the video card, if it is multirail and 450W, or low quality PSU. However, it seems more likely if that was the cause, the computer would shut off, not go to a black screen.

 

Re: Screen goes black. Bought new computer and installed video card and still screen

it may be that you are having driver issues cause i got the same comp and i installed a gt 620 then a 640 and the process was extremely hard and i finally figured it out and i believe windows 8 is the true source download the latest drivers and power off the comp and remove the card

plug video into onboard graphics then power on with nvidia card unplugged and when computer comes on open device manager put the card in then run driver installation and if it doesnt complete successfully thats cool just remove card restart and when computer comes on do it again but make sure you scan for devices and as soon as microsoft basic adapter pops up you start the installation right away