Ok, so I installed the latest Catalyst drivers for Radeon HD. However, I deselected on purpose the SDK package since I don't plan to use it.
After restart, BOINC couldn't connect to client and all GUI was blank. Nothing I did helped.
I've uninstalled completly the driver and installed the previous version without SDK.
Boinc was working but telling me that OpenCL support wasn't detected. After a google search, I found out that OpenCL support is in the SDK. I upgraded to latest drivers with SDK and BOINC now works without a hitch.
So, always install the SDK part for BOINC if you own an AMD Radeon HD graphics card...