When upgrading from sql server 2000 to 2005, a dbcc checkdb must be run at least once with data_purity.
If an error occurs with the data_purity, these rows should be fixed. Some fixes can be done by updating the data or by running dbcc updateuseage {table_name / index}, the error / message from checkdb will contain the table_name and/or index to run updateuseage with.