Help needed - error while adding geometry

Hi,
I'm a first time pgsql user. I've installed postgresQL and Postgis on a Windows XP Pro. machine. I just want to create a table and add geometry to it. So i tried the statements given in the manual.

CREATE TABLE gtest ( ID int4, NAME varchar(20) ); ( works fine)

SELECT AddGeometryColumn('', 'gtest','geom',-1,'LINESTRING',2); (this statement gives the blow error)

ERROR: function addgeometrycolumn("unknown", "unknown", "unknown", integer, "unknown", integer) does not exist
HINT: No function matches the given name and argument types. You may need to add explicit type casts.

Things that i tried:
(1) one of the help sites asks user to run
postgis.sql and spatial_reference_sys.sql files......
i seacrhed for postgis.sql file ..its not present in the installed directories. Also i copied the contents of spatial_reference_sys.sql to the sql prompt and tried executing it. It again throws an error.

Please can any one let me know whats the problem....is there some special way to install postgis on win XP. What all does one need to configure before adding geometry...

Thank you,
Ajay Kamath.

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