getting failed while creating user accounts

Hi all,
   I m trying to install postgres8.1 on my system.I
tried with account name postgres.I was getting an
error of unable to create user account.user account
already exists.
    If i try with any other user any of the following
error is arising

1.unable to create user account.user account already
exists.

2.Internal lookup failure.No mapping made between
accountname and serviceIDs

  I tried it a number of time wiht number of account
names and no of superuser names.I failed a number of
times
  How can i get installed with my postgres.If the user
accounts exists then where should i remove them so
that i can get that user name to be installed.

  Please give me the suggestions where i have to make
changes to make my installation to be
successful.Please reply me ASAP

Thanks and regards,
venu.p





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