Well, I am about to roll out about 3000 desktops of firebird 2.1 embedded.
After quite extensive testing I can state, unequivocally, that there is no
apparent logic to whether PCs need the 3 msvc(r/p/m)80.dll files.
Some PCs complain, some do not complain when they are missing. And I am
absolutely sure that I am using embedded 2.1 at all times in the testing.
This applies to NT4, XP and 2000. I can’t comment on Vista yet.

I don’t suppose someone knows the real facts here do they?

There MUST be registered entry points into older runtime libraries which
match the these newer ones. That's the only thing I can think of. On some
PCs where no complaint was made and embedded ran fine, later installs of the
libs reveal that they are indeed used when available. (i.e. you can’t delete
them cause the OS has loaded and locked them). But not installing them does
not raise any observable exception while running embedded (on some
machines). Very puzzling.

 regards
 Alan McDonald