It sounds like you've installed PG 7.3 on top of an existing PG 7.2 installation. The datafiles are incompatible.
The best thing to do would be to reinstall PG 7.2, use the PG 7.2 version of pg_dump to dump your database, then to install PG 7.3. Either initdb in a new data directory or move the old data directory someplace else and initdb in the default directory, then reload your data.
I think the PG docs may have some advice on how to upgrade that covers this stuff but am not 100% sure.