Cannot read block x of relation a/b/c: Invalid argument

Good afternoon all,

I'm attempting to run a pg_dumpall on an existing Win2000 SP4 server (v8.1.8) before upgrading to v8.2.4 and am running
intosome disconcerting error messages. 

The first time I did a 'pg_dumpall -s -c -U postgres -W', I received the following error messages:
pg_dump: SQL command failed
pg_dump: Error message from server: ERROR:  could not read block 33 of relation 1663/22873/1249: Invalid argument
pg_dump: The command was: LOCK TABLE public.tbl_population_descriptions IN ACCESS SHARE MODE
pg_dumpall: pg_dump failed on database "xxx", exiting

When I connected to the database using PGAdmin (v1.4.3), I received similar message (block 10 of relation
1663/22873/2609)

I know that oid(22873) is the database "xxx" (database name masked to protect the guilty!)

In attempting to find what these relations are I did a 'select * from pg_class where relfilenode=oid(1249) or
relfilenode=oid(2609);'and found that oid(1249) = pg_attribute and oid(2609)=pg_description 

My question is this: Is this an indication of a more catastrophic data loss/corruption/failure?

On a more important database, I'm receiving a similar error on oid(2663) = pg_class_relnode_nsp_index.

In connecting to each of the other databases on this server, I'm getting this error on pg_attribute in only one other
(notactively used) database. 


Thanks for any help.

Gord