installing postgresql on W2K; starting postmaster fails

I just installed cygwin including postgresql on my german W2K.
Cygwin runs fine and I managed to start the ipc-daemon and initdb.
I also made a user postgres as member of the administrator group
on the system and have run mkpasswd l > /etc/passwd to get the
actual password file for cygwin.

But starting postmaster e.g. with (postermaster D /test_data i) I get
The following error:

FATAL: could not set permissions on /tmp/.s.PGSQL.5432: Permission denied
/bin/postmaster: cannot create UNIX stream port

Of course I checked the permission for the /tmp directory but could not
Find anything obverse. I also could generate a simple text file using
Vim in the tmp directory. What permission does postmaster want??

Comments or solutions?

Thanks
Wolfgang Schoepp