Found this by accident, but it’s surprisingly easy. I’m using the latest IE, version 11.
Click the IE settings cog, and then click Manage add-ons from the menu.
In the Manage Add-ons dialogue box, highlight the add-on you want to find the information for, in this case, the HP Network Check Helper.
Click the More information link, to the bottom left. The More information box shows you the filename and containing folder of the file that “is” that add-on.
If you want to uninstall the add-on, sometimes it is as simple as running
regsvr32 /u <full path to dll>
(although that doesn’t actually work for the DLL example I’ve chosen in the examples above!)