INITDB problem

OK finally I started the ipc-daemon2 service
(I had to reinstall the last CygWin package)
Now when I try to initialize the data I get this
error:

$ initdb -D /usr/share/postgresql/data
The files belonging to this database system will be owned by user "mgonzalez
.
This user must also own the server process.

The database cluster will be initialized with locale C.

Fixing permissions on existing directory /usr/share/postgresql/data... ok
creating directory /usr/share/postgresql/data/base... mkdir: cannot create
directory '/usr/share/postgresql/data/base': Permission denied

I tried to run this process as the postgres user, however when I do:
$ su postgres
password:
su: incorrect password

Even if I change the password or left it blank it does not let me switch the
user.

Please help.

Thank you in advanced,

MAGO