SQL 2000? Typically means that someone's been messing around with the system tables, making direct updates without understanding the consequences of what they're doing.
Please run the following and post the full and complete results.
DBCC CheckDB (<Database Name>) WITH No_INFOMSGS, ALL_ERRORMSGS
DBCC CheckCatalog (<Database Name>) WITH No_INFOMSGS
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Gail Shaw
SQL Server MVP