problem getting started with PostgreSQL 8 beta 3 on XP

I installed Postgres 8 beta 3 on an XP box, with Postgres running as a
service. TaskManager tells me that postgres and postmaster are both
running. Using pgAdmin III, I can connect to the server and create
users, databases, tables, etc.

But at the command line, on the same machine where the service is
running, executing

createdb mydb

gives this:

could not connect to database template1: could not connect to server:
No such file or directory
    Is the server running locally and accepting
    connections on Unix domain socket "/tmp/.s.PGSQL.5432"?

The documentation says:

"This means that the server was not started, or it was not started where
createdb expected it."

Since pgAdmin has no trouble connecting, I assume that the server is
running and receiving connections at port 5432, so that leaves the
second part: not started where createdb expected it.

I've tried explicitly including the host, port, user, password, and that
doesn't help.

Any ideas what's going wrong here?

Steven Gollery
sgollery@cadrc.calpoly.edu