8.0.x windows installer fails?

[Win XP Professional 2002, SP 1, Pentium 4, 1.4 GHz, 512MB RAM]
I'm trying to install postgres 8.0.4 on my windows machine.
I downloaded ftp.us.postgresql.org/pub/mirrors/postgresql/binary/v8.0.4/win32/postgresql-8.0.4.zip.
I then double-clicked the file "postgresql-8.0".
It says "Welcome to the PostgreSQL Installation Wizard...".
The default english language is ok, so...
I click the "Start" button, and immediately get a window labeled "Windows Installer" saying:

This installation package could not be opened. Verify that the package exists adn that you can access it or contact the appilcation vendor to verify that this is a valid Windows Installer package.

So I tried the analgous file from 8.0.3, with the same results.
What am I doing wrong?

-- George Young
--
"Are the gods not just?" "Oh no, child.
What would become of us if they were?" (CSL)