I am having this very same issue but with a brand new and fully updated install of Windows 7. It was working on the previous installation of Win7 just fine, but I have no idea what is driving this current problem. Here are some specs and the data from the log:

Win7 Ultimate 64bit
AMD Radeon HD 5850 graphics card
amd-catalyst-15.7-with-dotnet45-win7-64bit driver set

BONIC Log, Startup
7/25/2015 10:45:38 AM | | Data directory: C:\ProgramData\BOINC
7/25/2015 10:45:38 AM | | Running under account Ron
7/25/2015 10:45:38 AM | | OpenCL CPU: AMD Phenom(tm) II X6 1090T Processor (OpenCL driver vendor: Advanced Micro Devices, Inc., driver version 1800.5 (sse2), device version OpenCL 1.2 AMD-APP (1800.5))
7/25/2015 10:45:38 AM | | No usable GPUs found
7/25/2015 10:45:38 AM | | Host name: ronultra
7/25/2015 10:45:38 AM | | Processor: 6 AuthenticAMD AMD Phenom(tm) II X6 1090T Processor [Family 16 Model 10 Stepping 0]
7/25/2015 10:45:38 AM | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 htt pni cx16 popcnt syscall nx lm svm sse4a osvw ibs skinit wdt page1gb rdtscp 3dnowext 3dnow
7/25/2015 10:45:38 AM | | OS: Microsoft Windows 7: Ultimate x64 Edition, Service Pack 1, (06.01.7601.00)
7/25/2015 10:45:38 AM | | Memory: 8.00 GB physical, 15.99 GB virtual
7/25/2015 10:45:38 AM | | Disk: 232.78 GB total, 106.76 GB free

I have read somewhere that there is an issue with the latest AMD Driver set that turns off OpenCL for the BONIC client but have no verification.

Any assistance would be appreciated.

Ron