Problem with a transaction on dump

Hello everyone,

It is the second time i got this error message (sorry, its french, i
prefer to c/p this one than making a bad translation) :

-------8<------------8<-----------8<-----------

pg_dump: ERREUR: Impossible d'accéder au statut de la transaction
892415538
DETAIL: Impossible d'ouvrir le fichier
«/var/lib/postgresql/7.4/main/pg_clog/0353» : Aucun fichier ou
répertoire de ce type
pg_dump: La commande SQL de sauvegarde du contenu de la table "facture"
a échoué : échec de PQendcopy().
pg_dump: Message d'erreur du serveur : ERREUR: Impossible d'accéder au
statut de la transaction 892415538
DETAIL: Impossible d'ouvrir le fichier
«/var/lib/postgresql/7.4/main/pg_clog/0353» : Aucun fichier ou
répertoire de ce type
pg_dump: La commande était : COPY public.facture ([...]) TO stdout;

-------8<------------8<-----------8<-----------

It say that it cant access to the transaction id 892415538 because the
file /var/lib/postgresql/7.4/main/pg_clog/0353 dont exist. And this file
doesn't exist.
I use PG 7.4.16 on linux debian etch. I already made a HD & memory test,
and i find nothing wrong.

Last time i got this error, the only solution was to get a "safe" backup
and lost some data. But i didnt have time to manage this server, so this
error is here for a couple of day, and im afraid about how many data may
be lost.

So if someone have an idea to have a "safe" backup and start on a clean
database...

Thx in advance,
Regards,