Invalid Page Header

Hi everybody,
I have a Postgres Database Server 8.0 in a Windows 2000 Professional.

I need to migrate this server to 8.1.4 or 8.1.5 Posgresql Version, but, when I execute backup command from pgAdmin III or from a command line (pg_dump [parameters] ) I get this error:

invalid page header in block 4686 of relation "tb_Bilhete"

Details:
==========================
pg_dump: ERROR: invalid page header in block 4686 of relation "tb_Bilhete"
pg_dump: SQL command to dump the contents of table "tb_Bilhete" failed: PQendcopy() failed.
pg_dump: Error message from server: ERROR: invalid page header in block 4686 of relation "tb_Bilhete"
pg_dump: The command was: COPY public."tb_Bilhete" ("inIdBilhete", "dtDataHora", "inDuracao", "vcNumRefPrivado", "vcNumRefPublico", "vcNumPartPrivado", "vcNumPartPublico", "chDirecaoRef", "vcTronco", "chInterno", "chProcessado") TO stdout;
pg_dump: *** aborted because of error
Process returned exit code 1.
==========================

How Can I resolve this problem?
I have other backup, but I need the more recent backup.

I search at the internet about this problem, but I find solutions only for Linux System using Linux tools.

Thanks
André Guergolet