Johanna,

j> LibraryName=fbclient.dll
j> DBName=localhost/3121:C:\xxx.FDB
j> CharSet=ISO8859_1
j> SQLDialect=3

j> I start the Firebird 1.5 Server with "fbserver -a -i -p 3121"

This means it is starting as an application with the privileges of the
launching user, which ordinarily cannot write under the system root in
Vista (neither on WinXP, for non-administrative user accounts, AFAIR).
Just find a more appropriate place to put your databases, like
somewhere under your user home folder.

Ciao
--
Nando Dessena
======================================================
I support Firebird, I am a Firebird Foundation member!
Join today at http://www.firebirdsql.org/ff/foundation
======================================================