Postmaster is starting but shutting when trying to connect (Windows)

Hello,

i have a PostgreSQL (8.1) installation for testing purposes which was
running fine for several months now (Windows XP). I was working with it
yesterday, and today after booting my computer and restarting the
service (I'm starting the service manually, because I don't need the
server running all the time) postmaster comes up fine, no entries in the
log file.

But as soon as I try to connect to the server (does not matter what type
of client, psql, PgAdmin or JDBC) I get an error message in the log
file. Here is the full log file:

2006-07-07 09:18:15 LOG: database system was shut down at 2006-07-07
09:14:00 Westeuropäische Sommerzeit
2006-07-07 09:18:15 LOG: checkpoint record is at 0/3C82F60
2006-07-07 09:18:15 LOG: redo record is at 0/3C82F60; undo record is at
0/0; shutdown TRUE
2006-07-07 09:18:15 LOG: next transaction ID: 100576; next OID: 19416
2006-07-07 09:18:15 LOG: next MultiXactId: 1; next MultiXactOffset: 0
2006-07-07 09:18:15 LOG: database system is ready
2006-07-07 09:18:15 LOG: transaction ID wrap limit is 2147484148,
limited by database "postgres"

2006-07-07 08:43:54 LOG: incomplete startup packet
2006-07-07 08:44:18 LOG: could not receive data from client: An
operation was attempted on something that is not a socket.

2006-07-07 08:44:18 LOG: incomplete startup packet
2006-07-07 08:45:27 LOG: received fast shutdown request
2006-07-07 08:45:27 LOG: shutting down
2006-07-07 08:45:27 LOG: database system is shut down
2006-07-07 08:45:28 LOG: logger shutting down

I already checked with netstat whether I have something running on port
5432 and I changed the port in posgresql.conf just to make sure it's not
cause by another process that locks port 5432 that I'm not aware of. But
still the same thing

There is no firewall active which would block that port either (and - as
I said before I shut down my computer yesterday evening it was working fine)

There is also no postmaster.pid file hanging around when I shut down the
service.

Any hints what I could try to fix this?

Thanks in advance
Thomas