Today, I redownloaded the zip package (win32 build) of firebird 1.5.1
from firebird.sf.net.

and followed the instructions of docs/README_installation.txt.

it works now.

BTW, I always installed the exe package before and tried to make the CS
mode work but failed again and again. I have also tried the instreg.exe
and instsvc command line tools. Now, I am sure that some wrong
operations must occured before some days.