After extracting the files, it appears to check for a previously installed version, and states that it found a *newer* version installed, and requires that it be uninstalled before installation will continue.
I did find what appears to be the remnants of a previous install of an Intel OpenCL SDK - a directory structure without most of the usual files in it. But nothing in the Control Panel "uninstall programs" dialog. I deleted the empty directory tree of the SDK, and it still thinks there is a previous newer installed. (Possibly the previous user of this box had an Intel internal alpha SDK installed that is newer?)
I do have a graphics driver installed that is newer than the SDK, but deleting the opencl.dll that comes with that does not let me install the SDK.
What triggers this "newer version" message/behavior? A registry entry? Is there something I can do to get past it?
The OpenCL SDK installer has detected that never version of Intel SDK for OpenCL- CPU Only Runtime Package is installed, which is not SDK (and is part of SDK and GFX Driver). The newer version of CPU Only Runtime Package comes with newer version of GFX driver in your case. For downgrade CPU Runtime Package you must remove it from Programs and Features, but if it is not present in list, it means that you have broken its removal mechanics, which can happen if remove related to it files from program files directory manually. For fix this you will need to removing the HKLM\Software\Wow6432Node\Intel\OpenCL and HKLM\Software\Intel\OpenCL keys.