Problem with creating triggers in PostgreSql

I am facing a problem when creating a function in
PostgreSql 7.2.1.
I get the following error :

ERROR: stat failed on file
/usr/local/pgsql/lib/plpgsql.so

I think I require plpgsql.so file for this.
Where can I get this file.
I am using windows Operating system.

Thanks and Regards,

Kallol Nandi

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