Having trouble installing on Vista

Hi,

When trying to install PostgreSQL on Windows Vista with
postgresql-8.3.7-1-windows.exe, I am asked for a password for the
postgres-user. But when I give one, it says it is incorrect.
Ergo, the user exists and I don't have the correct password. How can I
find the user and change the password? Because when I look in
Useraccounts, there's only me. No other accounts.

Thanks in advance!