On Vista:
Click on Start
In the little search box in the start menu, type "compmgmt.msc"
A window by the name "Computer Management" comes up.
There is a pane on the left, where the top node is called "Computer management', expand it, go to "Local users and groups", expand that and then click on "users", this will bring the list of "users" in the right pane. You should see the "postgres" user in this list. Right click on the postgres user and it will show option for "setting a password". Additionally, from this menu, you can delete the user as well.