You now have BOINC 7.2.38?
Could you please show its start up messages?

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There are no starup messages since I enabled the service.
let me try something, it will take a few minutes, stay on the post I'll upload come pictures

So, uninstall BOINC through Control Panel\Programs\Programs and Features and reinstall it, this time uncheck "Service Install" in the advanced section of the third screen of the installer.

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Hello back

There are many problems with the Catalyst and the driver. I had to uninstall some softwares with Safe mode, I deleted the driver Windows was using (from AMD) and I have now the basic standard driver.

I am uninstalling Catalyst and all APP-related softwares. I'm gonna download the latest driver from AMD right now

EDIT : I already uninstalled BOINC
EDIT 2 : Do you recommend me a specific version for the driver ? I'm downloading Catalyst 13.12

You have 1 minute to aswer my question ^^ Driver will soon be downloaded

Don't ask me - I'm exclusively NVidia.

But a late-model driver sounds right for a late-model card.

Yup.
So, I installed the driver. Let's install BOINC without the service

EDIT 1 : I installed BOINC v7.2.39 (latest BETA)... For now, it doesn't start with an error and have detected my GPU

EDIT 2 : YES !! It works !! My GPU completed a task in 36 seconds instead of 2 hours with my CPU

EDIT 3 : How I've done
1. Uninstall BOINC and restart your computer in Safe Mode
2. Uninstall AND DELETE your video card driver from the Windows' Device Manager.
3. Start in normal mode and uninstall all the Catalyst components and then download the latest one via the official website
4. Delete any traces of ATI Technologies in Program Files (x86 and x64) and installer traces (AMD folder in C:\)
5. Install the ATI Catalyst you just downloaded
6. Delete any traces of BOINC in ProgramData and Program Files and reinstall it (without service)

This worked for me.

EDIT 4 : now I have sooo many tasks xD

Do check with your project - Milkyway? - that the tasks you completed in 36 seconds have been accepted as valid results.

Yup, the tasks are valid. Everything is fine. But I have one last question : why does Milkyway is the only project that use my GPU ?

Thanks,
- Europa

EDIT 1 : http://milkyway.cs.rpi.edu/milkyway/results.php?userid=553645&offset=0&show_names=0&state=4&appid=

That link is not usable by other people, unfortunately.

Try http://milkyway.cs.rpi.edu/milkyway/show_host_detail.php?hostid=557409 instead.

You might like to revisit your parallel thread on the MilkyWay forum.