>How do you know whether you're supposed register an OCX, DLL, or EXE when it gets installed into the Windows System or WinNT\System32 folder?

Any OCX or OLE-Enabled .DLL, as well as any Server .EXE, must be registered; .DLLs need to be present in the system path, which normally means installing them to the directry resolved as the system directory by Windows.