trouble connecting to DB

Hi - I installed PostgreSQL 8.4 on Windows XP Pro using the packaged
installer with no errors, but am having a problem connecting. The symptoms
are:

1. When attempting to connect in PGAdmin III, I get an error message:
ERROR : Error connecting to the server: server closed the connection
unexpectedly
This probably means the server terminated abnormally
before or while processing the request.

2. The Windows Application and System event logs show no unusual errors
just: 'Server Started and accepting connections'.

3. When I start the server, I get something like the following in the
postgresql log (in \pg_log):

2009-07-18 22:34:37 MSTLOG: database system was shut down at 2009-07-18
22:34:33 MST
2009-07-18 22:34:37 MSTLOG: database system is ready to accept connections
2009-07-18 22:34:37 MSTLOG: could not receive data from client: An
operation was attempted on something that is not a socket.
2009-07-18 22:34:37 MSTLOG: incomplete startup packet
2009-07-18 22:34:37 MSTLOG: autovacuum launcher started
2009-07-18 22:34:39 MSTLOG: could not receive data from client: An
operation was attempted on something that is not a socket.
2009-07-18 22:34:39 MSTLOG: incomplete startup packet

(The incomplete startup packet could repeat more times.)

Usually when I've seen the 'connection closed' message with other software,
it means an authentication error, so I've tried the following with no luck:

1. Verified the postgres user-id in Computer Management. It is a member of
no groups (seems strange to me, but apparently this is required.)
2. Verified the password by changing it in Computer Management/Users, and in
Services 'Login as' to be sure they matched and were what I thought.

3. I've also checked firewall (Comodo Pro) and antivirus (Avira) logs -
there are no entries showing postgres being blocked.
4. Also attempted the connect with firewall and antivirus turned off.
5. Since the postgres service is dependent on the RPC service, I verified it
was set to Automatic and running.

This is probably something simple and obvious, but I don't see it. Any help
would be appreciated.

Thanks,
Terry S. in AZ