Postgresql 7.3.2-3 and 7.3.3-4 freeze

Hi, first of all i don't speak english very good. I make a function in pl/pgsql, that cause a denial of service. The function has a simple select. If anyone can help with this.

create table "Users" (
"Usu_ID" int, "Usu_Nick" varchar(32), "Usu_Apellido" varchar(50), "Usu_Nombre" varchar(50),
"Usu_Password" varchar(32), "Usu_Email" varchar(64), "Pai_ID" varchar(2), "Usu_FechaNac" date,
"Sex_ID" smallint, "Usu_TipoDoc" varchar(20), "Usu_NumDoc" varchar(20), "Usu_Direccion" varchar(50),
"Usu_CodPostal" varchar(16), "Usu_Ciudad" varchar(35), "Usu_Estado" varchar(35), "Ocu_ID" smallint,
"Ind_ID" smallint, "Usu_FechaInscrip" timestamp with time zone, "Usu_Codigo" varchar(8),
"Usu_Observ" varchar(100), "Usu_Jue_ID" smallint, "Usu_Newsletter" varchar(1), "Usu_Activo" varchar(1),
"Usu_GMT2" varchar(3), "Fmt_ID" smallint, "Usu_Ocu_Other" varchar(50), "Usu_Ind_Other" varchar(50),
"Idi_ID" smallint);

CREATE FUNCTION alta_usuario(varchar, varchar, varchar, varchar, varchar, varchar, smallint, smallint, date, smallint, varchar, varchar, smallint, varchar, varchar, varchar, varchar, varchar, smallint, varchar, smallint, varchar) RETURNS int4 AS '

DECLARE
xUsu_Nick ALIAS FOR $1;
xUsu_Apellido ALIAS FOR $2;
xUsu_Nombre ALIAS FOR $3;
xUsu_Password ALIAS FOR $4;
xUsu_Email ALIAS FOR $5;
xPai_ID ALIAS FOR $6;
xFmt_ID ALIAS FOR $7;
xIdi_ID ALIAS FOR $8;
xUsu_FechaNac ALIAS FOR $9;
xSex_ID ALIAS FOR $10;
xUsu_Codigo ALIAS FOR $11;
xUsu_Newsletter ALIAS FOR $12;
xUsu_Jue_ID ALIAS FOR $13;
xUsu_GMT ALIAS FOR $14;
xUsu_Direccion ALIAS FOR $15;
xUsu_CodPostal ALIAS FOR $16;
xUsu_Ciudad ALIAS FOR $17;
xUsu_Estado ALIAS FOR $18;
xOcu_ID ALIAS FOR $19;
xUsu_Ocu_Other ALIAS FOR $20;
xInd_ID ALIAS FOR $21;
xUsu_Ind_Other ALIAS FOR $22;

Comprueba int4;
BEGIN
IF xUsu_Nick IS NULL THEN
RETURN 0;
END IF;
Comprueba = (SELECT "Usu_ID" FROM "Users" WHERE LOWER("Usu_Nick") = LOWER(xUsu_Nick) );
IF Comprueba IS NOT NULL THEN
RETURN 0;
END IF;

RETURN 1;

END;' LANGUAGE 'plpgsql';

select alta_usuario('cerito','a','a','a','a(at)a(dot)com','AR',1,1,'1981-07-24',1,'a','a',1,'-3','a','a','a','a', 3, '', 4, '');

Regard,
Tavella Gaston.