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.