The file is actually MSVCRT.DLL, a standard microsoft library not related to
firebird.
I had the same problem installing 1.5 on NT. The DLL version needs to be
version 6.0.8397.0 or better.
Do a search on the machine for duplicates of that file, and see if any of
them are a newer version. You will need to rename the old DLL file in DOS
before replacing it with the newer version and rebooting. After that things
work fine.

Download the correct version here

http://www.dll-files.com/cgi-bin/cgiwrap/dll-files/topdown/download.pl?file=msvcrt.zip==

if you can't find it.

Regards
Gary