When I browsed to c:\Windows and looked for Installer I didn't see it. So I typed c:\Windows\Installer in Explorer's address field and it came up. I selected all the folders, right clicked and chose "Properties."
There was almost 30GB there!
It turns out that c:\Windows\Installer is used to uninstall programs. However in an old, active system (like X201) these files tend to get orphaned.
Google it if you like but there's much confusion out there. Most of the discussions wander off into telling the poster about how to run Disk Clean-Up to delete old versions of Windows. This is NOT the problem.
I saw PatchCleaner mentioned again and again. This article describes it best. PatchCleaner's home page is here and there's a download link but SourceForge has a project that has created a portable version.
I downloaded the portable version and ran it on X201. As I suspected it found over 26GB of orphaned files. I chose the option to have PatchCleaner move these orphaned files over to my Drobo.
Problem solved. I'll let you know if this created any new problems.