You can copy&paste passwords into the steam-login window with a very simple hack. Browse to Steam/Public and open SteamLoginDialog.res with notepad or wordpad. Goto "PasswordEdit" and change between { and } "textHidden" "1" to "textHidden" "0" to show the password. Then add or change "tabPosition" "0" to "tabPosition" "1" and you will be able to copy&paste passwords!
are you sure that you edited the right file (there are many different skins) and that you have administrative permissions to save? In vista, since it is located in /Program Files/ you can't save anything there without administrative permissions.l