I did some more looking around, and when I delete riched20.dll, it automatically reappears after a few seconds.
Windows File Protection is kicking in. To get Windows to even show the dllcache folder, you have to enable viewing of hidden and system files, and also enable viewing windows system files, both options are in the Folder Options dialog. (Sorry, not near a Windows box to find the exact text of these options).
Windows 2000 was the first Windows OS to really have this agressive restore, while 98 had System File Checker (sfc.exe) to detect problems manually.