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.