Really odd corruption problem: cannot open pg_aggregate: No such file or directory

So, one of the many machines that I support seems to have developed
an incredibly odd and specific corruption that I've never seen before.

Whenever a query requiring an aggregate is attempted, it spits out:
cannot open pg_aggregate: No such file or directory
and fails.

If I do:
select * from pg_class where relname='pg_aggregate';
I see that the relation exists.

If I check the relfilenode in the data directory, that exists, and
seems to be an object file containing what should be the basic
aggregate functions.

version: PostgreSQL 7.2.3 on i686-pc-linux-gnu, compiled by GCC gcc (GCC) 3.2 20020903 (Red Hat Linux 8.0 3.2-7)

The system ran for a few weeks before anything odd happened, and
then suddenly this. Does anyone have any ideas? Now that I look at
the above string, I realize that the system /is/ an Athlon processor.
Does anyone know if there could be an issue between the i686 and
athlon optimizations?

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