How do I reset the postgres password for PostgreSQL on Windows?
From the 9.0 FAQ, I lost the database password. What can I do to recover it?:

I lost the database password. What can I do to recover it?
You can't. However, you can reset it to something else. To do this, you

Edit pg_hba.conf to allow trust authorization temporarily - go here for its location
Reload the config file (pg_ctl reload)
Connect and issue ALTER ROLE postgres WITH PASSWORD 'newpassword'; to set the new password (for first time users note the semi-colon)
Edit pg_hba.conf again and restore the previous settings
Reload the config file again