unable to (re-)install

I All.
I've installed postgres 8.2 on my Windows XP. All Ok. Then I've removed this
installation from the control panel. Then I've tryed to reinstal the same
version of postgres but the first time I've received the error: "the user
postgres already exiists", then after the change of the user name I've
received the error: "Internal account lookup failure: Non è stato effettuato
alcun mapping tra i nomi di account e ID di protexione" (My system is
italian, in english this message should be: "No mapping has been performed
between account names and protection ID").

Any idea of what I'm doing wrong?

Luca.
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