6.6.x versions of BOINC have separate GPU scheduling so at times you see BOINC requesting GPU tasks and receiving 0 (obviously as PrimeGrid and the majority of BOINC projects do not have GPU applications/work). Of course BOINC doesn't know that and will keep requesting GPU work for non-GPU projects, something I also found annoying. Note that any PrimeGrid work you did actually download was CPU work and was not processed by the GPU that wouldn't be possible.
To make BOINC stop using GPU and get rid of these GPU download requests (provided you are not doing work for any of the other GPU/CUDA projects) you need to do the following:
Find your BOINC Data folder (Vista default is C:\ProgramData\BOINC) and in there create a file named cc_config.xml
Use Notepad or another text editor and inside the file write the following:
Save the file, shut down BOINC client and manager.
Next time you start BOINC you should see a message in the BOINC Manager Messages tab saying "Configured to not use coprocessors"
That means BOINC will ignore your GPU. Plus using this way you keep the latest GPU drivers without the need to roll-back or uninstall anything.