Holy cow, I wrote a book!
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.
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
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.)