So you installed PostgreSQL by itself prior to PokerTracker 3? I think it will be more hassle to uninstall/reinstall as that leads to issue IMHO.
Lets try removing the need for a password then using the "postgres" username.
Go Start->Program Files->PostgreSQL 8.2->Configuration Files->Edit pg_hba.conf
Scroll to the bottom of the conf file and change the two lines that contain "md5" to "trust".
File->Save. Restart PostgreSQL (or your computer). Then when you are prompted to enter a username, use "postgres" and omit the password field.
Best regards,
Derek