Windows 7: computer shuts down at windows loading screen

Most likely a virus.
Boot in to safe mode and run some scans.

"Safe mode is a troubleshooting option for Windows that starts your computer in a limited state. Only the basic files and drivers necessary to run Windows are started.
If an existing problem doesn't reappear when you start in safe mode, you can eliminate the default settings and basic device drivers as possible causes."

If Safe mode works then you can first assume that some device (hardware) or device driver (software) is faulting in Normal Mode. The first suspect of course would be the new video card. Safe mode runs with standard VGA video drivers and does not start the advanced features of the card.

When replacing a video card it is important to completely remove all of the old drivers and graphics programs that come with the drivers before installing the new card. Did you uninstall the drivers, PhysX, and 3D Vision for the 4850 cards?

Do you still have the 4850's? To test the new video card you could uninstall all the existing drivers from Safe Mode, shut down, then install one of the the 4850's again to see if the machine will boot with the old (known working) card.

Use these instructions to replace a video card. Some steps (like Driversweeper) will need to be skipped because you are in a no-boot situation. But if you do get into normal mode and want to install the new 6870 card then it would be a good idea to follow all the instructions.

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To change a video card, particularly if you are changing brands, it is very important to follow proper procedure, as follows:

• Go to the ATI or nVidia website and download the most current known good driver and save it to an easy to find location.
• Now go into Windows Explorer and in the C: drive find the nVidia or Program Files > ATI folder (inside will be Drivers > your driver version) and delete it (the whole folder).
• Go into Start > Control Panel > Remove a Program and uninstall all programs for the video card. For nVidia, do PhysX and Stereoscopic 3D first, then the driver. The control panel will uninstall with the driver. For ATI, select “ATI Catalyst Install Manager” and click on “Change”, then “Uninstall All Components”. Do not restart the computer at this time.
• Run DriverSweeper to uninstall all video remnants Guru3D - Driver Sweeper .
• Shut down computer and disconnect power.
• Remove the old nVidia/ATI card, install the new ATI/nVidia card. Make sure all auxiliary power connectors are plugged in. Some cards need 2. Read the manual.
• Reconnect the power and boot the computer.
• When it reaches the desktop Windows will find new hardware and will install it's own WDDM1.1 driver. Let it. You will be asked to restart. Do it.
• Once back on the desktop you can now install the nVidia/ATI driver package for your card.

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Unfortunately, you could have some other damage to your system caused by the overheating issue. But let's start with the video cards, and hope that is all it is.

Let us know your results.

The black screen while booting usually is a harware issue but hard to track down. Follow Tveblen's steps, if that is no help try removing all but one stick of memory.
If you have a new PC case you may have forgot a connection or one isn't fully seated, so double check all of them. I've forgot to connect the EPS 12V before, so the PC was dead.

This doesn't sound like a virus or infection.

problem solved. i feel kinda silly but im new to the whole dual desktop thing. it was doing what it was because i had plugged in a 22 hdtv to the hdmi port as well on my graphics card and the settings in the amd vision control center weren't configured right.

the computer works great now! thanks for everyones help lol

Doesn't that just kill you? We all do it from time to time.

Glad to hear you got it sorted. Have fun!