hi problem with installing postgresql8.1

Hi all,
    I am new to pgsql family.I had a problem with
installing postgres8.1 i was getting the following
error at the 4th step of installation the error is as
follows:

Data directory error:
  The specified data directory is not empty

   If you have an existing database with teh same
major version number,you do not need to initialise a
database cluster.
   If you have an existing database with the same
major version you need to backup your old database and
create a new one.

   why this warning actually arises.If  I install by
unchecking the initialise database cluster then the
next three steps of installation procedure is not
shown.It goes directly installation and starts
installation after some time a fatal error arises
saying accont not matching....................
    and rolls back.i have tried it with lots of
account names and lot of super user names.The steps i
was talking about are as per the given link.


http://pginstaller.projects.postgresql.org


    Can any one give the solution to this
problem.Please suggest solution to overcome this
problem ASAP.

Thanks in advance,
Venu.



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