After pulling an HD5850 from a system that also had a gtx460, opencl would not work even though the utility "gpu cap viewer" said it was available and it had worked on the other gtx460 before the ATI card was pulled.


The above utiity has an "OpenCL demos" popup that would not run claiming that I "did not have a platform that supported open cl". MilkyWas also said I had no opencl.

The above demo, and MilkyWay, worked fine after I uninstalled the ATI video driver. Obviously, I should not have a driver for a missing video card. However, it seems to me that a 3rd party program should be able to determine if opencl is available for nvidia even if it is not available for ati.

my 2c tells me there are a lot more important things to fix beside this.

Uninstall the Nvidia drivers, then reinstall the Nvidia drivers,

Both Nvidia & AMD/ATI drivers install OpenCL.dll, if you run a computer with both Nvidia and AMD/ATI drivers, then you uninstall a driver (from one or the other) OpenCL.dll gets uninstalled (if the uninstaller is working correctly)
(The order of driver installation is also important when running a mixed Nvidia/AMD/ATI computer, if you install Nvidia drivers last, some AMD/ATI OpenCL apps give inconclusive results, so AMD drivers should be installed last)

Claggy

Added to what Claggy said, it's also better to uninstall the drivers for the videocard that you removed. Less chance for Windows to get confused.

In the case of OpenCL.dll, it's installed by a lot of OpenCL compatible hardware drivers, one being a little newer than the last. Whichever driver you installed last overwrites the file with a version of its own, whether that version is newer or not.

Normally when you uninstall that last driver, Windows should restore the files that were overwritten to its original versions. But this hangs a lot on what the installer/uninstaller does. Not all of the uninstallers delete their own files, or instruct Windows that they've left the building and that original files should be restored.

Which is why we also always advice to run something like Driver Sweeper to remove all the remnants of an old driver that the uninstaller forgets about. Reinstalling the drivers for the other piece(s) of hardware isn't a bad thing either. At least then the OpenCL library is the one that's pointing at your videocard.

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