I have a 7870 running XP and have the same problem. No OpenCL devices. have tried all the different driver versions I could get my hands on, and the different SDK's to no avail. Going to boot a linux and see how that works
I had same problem, and no solution for it. I gave up trying to work with XP.
Try with trial version of Windows 8 Enterprise Edition (90 days, possibility of 180 days). There everything works ok.