Depending on what version of Windows you're running, there may be a variety of support DLLs for things that aren't formal product components, but which are merely along for the ride. For example, Windows 95 came with MFC30.DLL because the Fax Viewer was written with the help of MFC 3.0. But if you look at Windows 98, MFC30.DLL is gone.
What happened is that Windows 98 didn't have a fax viewer that used MFC 3.0. The fact that some MFC 3.0 DLLs wound up on the machine with Windows 95 was merely a side effect of the implementation and not a part of the product specification. And in fact, if you chose not to install the Fax Viewer during Windows 95 setup, you wouldn't have gotten MFC30.DLL at all either, because MFC30.DLL wouldn't have been needed.
We had a category of Windows 98 compatibility bugs from programs that assumed that MFC30.DLL was a contractual part of Windows. If the tester did a minimal install of Windows 95 and then installed the application, the application wouldn't run there either. The application vendor simply assumed that MFC was part of Windows, even though it wasn't. In other words, the program didn't work even on Windows 95. Is it a bug in Windows 98 if the program didn't work on Windows 95?
This problem persists today. People go scrounging around the binaries that come with Windows looking for something they can remora. And then they're surprised when those binaries change or vanish entirely. For example, one customer had reverse-engineered the Kodak image viewer in Windows 2000 and wanted to keep using it in Windows XP. But those components are not included in Windows XP; the Kodak image viewer was merely a stopgap solution until the Windows XP fax and image viewer came along. (The linked Knowledge Base article has more information on that product.) I mention this because that customer, a Fortune 500 company, was trying to copy the files from Windows 2000 and install them onto a Windows XP machine and actually came to us asking for help in what may very well have been a violation of the Windows license agreement! (And certainly a violation of Microsoft's agreement with Kodak.)
(I now predict that everybody will comment on the last two sentences and completely ignore the point of this article.)