Getting the "no usable GPUs found" message.
I just switched a GPU out on one machine from a GTX460 to a HD5850. I uninstalled all drivers rebooted and installed Catalyst 12.2 with open Cl drivers. Tried 14.4 first with no joy.
Computer shows 5800 GPU with drivers working properly.
BOINC was running GPU units just fine prior to all this.
Tried uninstalling and re installing BOINC.

Any thoughts.

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You obviously are not a newcomer to Boinc, but have you seen this?
http://www.hal6000.com/seti/boinc_ati_gpu_cheat_sheet.htm
Perhaps a downgrade to Catalyst 12.1?

Catalist 12.2 for Windows XP doesn't have OpenCL drivers, even though the download page when it was first released claimed that it did.

When I unpacked the installation package to my machine, this file existed:

C:\AMD\Support\12-1_xp32_dd_ccc\Packages\Apps\OpenCL\OpenCL.msi

See if you can find it in your C:\AMD\Support\12-2_xp32_dd_ccc tree.

Thank you both for the info, I will drop back to 12.1 and see if that works for me. Looks likely that is my issue.

Problem solved! Your solution was the trick. Thanks!!