WARNING - the latest round of 64-bit Windows 10 updates appears to have blocked BOINC from finding the OpenCL interface to my GT 440. The CUDA interface does not seem to be affected.

Solution found if you have GEFORCE Experience installed - start it. Either tell it to install a newer driver, or click on the three dots to the right of the message telling you which driver you have installed, and tell it to reinstall the driver.

This tells it to install the Nvidia version of the driver, with the OpenCL interface, instead of the Microsoft version, which usually does not have the OpenCL interface.