Installing postgres on win XP

I am trying to install Postgresql 8.0.3/ 8.0.2 on windows XP.

I get following error:

This installation package cannot be openned. Verify that the package exists
and that you access it, or contact the application vendor to verify that
this is a valid windows installer package.

This happens right after I click on "Start>" button after the language
choice window.

Appreciate your help.  Thanks,

Andy