Sorry if this addressed elsewhere, but I can't see it.
I have Windows8 64, an AMD 6570 and Catalyst 13.1 and can't seem to use the GPU.
The log file that seems relevant is

20/02/2013 23:46:44 | | No config file found - using defaults
20/02/2013 23:46:44 | | Starting BOINC client version 7.0.28 for windows_x86_64
20/02/2013 23:46:44 | | log flags: file_xfer, sched_ops, task
20/02/2013 23:46:44 | | Libraries: libcurl/7.25.0 OpenSSL/1.0.1 zlib/1.2.6
20/02/2013 23:46:44 | | Data directory: C:\ProgramData\BOINC
20/02/2013 23:46:44 | | Running under account Tim
20/02/2013 23:46:44 | | Processor: 2 GenuineIntel Intel(R) Pentium(R) CPU G860 @ 3.00GHz [Family 6 Model 42 Stepping 7]
20/02/2013 23:46:44 | | Processor: 256.00 KB cache
20/02/2013 23:46:44 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 cx16 sse4_1 sse4_2 syscall nx lm vmx tm2 popcnt pbe
20/02/2013 23:46:44 | | OS: Microsoft Windows 8: x64 Edition, (06.02.9200.00)
20/02/2013 23:46:44 | | Memory: 7.94 GB physical, 8.44 GB virtual
20/02/2013 23:46:44 | | Disk: 65.41 GB total, 16.79 GB free
20/02/2013 23:46:44 | | Local time is UTC +0 hours
20/02/2013 23:46:44 | | ATI GPU 0: AMD Radeon HD 6x00 series (Turks) (CAL version 1.4.1741, 1024MB, 991MB available, 1248 GFLOPS peak)
20/02/2013 23:46:44 | | App version needs OpenCL but GPU doesn't support it
20/02/2013 23:46:44 | SETI@home | Application uses missing ATI GPU
20/02/2013 23:46:44 | SETI@home | URL http://setiathome.berkeley.edu/; Computer ID 6897699; resource share 100
20/02/2013 23:46:44 | SETI@home | General prefs: from SETI@home (last modified 10-Feb-2013 16:50:51)
20/02/2013 23:46:44 | SETI@home | Computer location: home
20/02/2013 23:46:44 | SETI@home | General prefs: no separate prefs for home; using your defaults

Can anyone please tell me what to do?

It is a bug in the BOINC version that you are using.

Lets try the latest alpha build and see if it shows the OpenCL options.
http://boinc.berkeley.edu/dl/boinc_7.0.52_windows_x86_64.exe

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You might also want to back down to ATI driver 12.8 The 13.1 has a bug that kills the OpenCL process.

If you feel lucky, you can uninstall the OpenCL from the 13.1 and then reinstall the 12.8 OpenCl

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In a rich man's house there is no place to spit but his face.
Diogenes Of Sinope

I tried the 7.0.52 version of BOINC, that seems to have done the trick! Thanks very much

I've had a similar problem for almost 6 months, and have tried many different combinations of BOINC versions, and CCC versions. I'm wondering did you switch to Catalyst 12.8 or did you stick with 13.1 after you updated to BOINC 7.0.52?

Mike has told me that the 13.1 drivers are broken for the ATI/AMD GPU apps. You can uninstall your old driver, install the 13.1 without the sdk development driver set, then install the 12.8 sdk. Seems a bit convoluted but it does work

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In a rich man's house there is no place to spit but his face.
Diogenes Of Sinope