initial windows install : can't createuser or createdb - what user / pass to use?

I've just installed postgresql on windows version 8.0.0 rc-2 and am
trying to do very first things like createuser or createdb but all I
get is messages like:

createuser: could not connect to database template1: FATAL: password
authentication failed for user "xxxxx"

How do I add users or do other admin stuff at startup? Is there a
default administrative user and password that I should know about?

From the example below, lbaigell is the user I log into windows with -
so Postgres knows about that user name by itself - the password I'm
trying to use is the windows password - a 6 alpha character string.

Thank you
Henry

interaction below:

C:\Program Files\PostgreSQL\8.0.0-rc2\bin>createuser
Enter name of user to add: mkon
Shall the new user be allowed to create databases? (y/n) y
Shall the new user be allowed to create more new users? (y/n) y
Password:
createuser: could not connect to database template1: FATAL: password
authentication failed for user "lbaigell"

C:\Program Files\PostgreSQL\8.0.0-rc2\bin>c