Character set error

Good Day.

I´m working with postgres 7.4. on Win XP and when I do a query like

select* from table

it returns me this error message:

Invalid character data was found. This is most likely caused by stored data
containing characters that are invalid for the character set the database
was created in. The most common example of this is storing 8bit data in a
SQL_ASCII database.

I know it´s a character set problem, I really have SQL_ASCII encoding but I
don´t know how to fix it

Any idea will be welcome

Thanks

Mauricio Fernández A.
Ingeniero de Sistemas
U. Autónoma de Manizales