When I get some new work units and my CPU is already computing, at the unit's status there is written : "0,08 CPU + 1.0 GPU". And then, immediately, it says Computation error. So, I think there might be something wrong with my GPU, but I don't really know what.

Also, I do 2,5GFlops/s , with a AMD Athlon 7550 DualCore, at 2.5GHz + a Radeon HD 3400 Series. So for this specs 2,5GFlops seems kind of low... (at a WU with computation error, in which the GPU was involved, the speed is estimated at 28GFlops/s).

Also, I think I should specify that I run the PrimeGrid project, on Windows XP SP2.

Can someone please help me out, so that I will not get the Computation Error thing, and to be able to use my GPU? ( I have the latest drivers downloaded from the ATI website (although I am not quite sure of that) )

Does this thread help? If not ask on the primegrid forums - better chance of help.

ÂTI seems to have dropped OpenCL support for HD 4xxx and below with Catalyst 11.6 , see http://www.primegrid.com/forum_thread.php?id=2683#38629. So either downgrade your GPU drivers or give ATIs OpenCL SDK a try.

I really don't have a clue on how to do that. Can you please help me out?

go to http://support.amd.com/us/gpudownload/windows/previous/Pages/radeonaiw_xp.aspx
Download Catalyst 11.5. Uninstall all your current ATI driver software (Catalyst, ...). Reboot. Install the downloaded Catalyst 11.5. Reboot. Hope that PPS sieve is running again.

BTW: Please upgrade yor system to XP SP3 (download from Microsoft), especially if you aren't aware of possible risks. Enable automatic updates to get most important hot fixes.