It seems that SQL Server database or the database backup file get corrupted. Try following steps to resolve this error:
1. To check the SQL Server 2000 database integrity, run the command "DBCC CHECKDB(database_name)" on the SQL Server 2000 instance. 2. If DBCC CHECKDB execution print no error, try to backup and restore the database again. 3. If DBCC CHECKDB execution prints error messages, you switch the database in single-user mode and run DBCC CHECKDB with a repair option(REPAIR_ALLOW_DATA_LOSS | REPAIR_FAST| REPAIR_REBUILD). Note that using REPAIR_ALLOW_DATA_LOSS can result in risk of data loss. For more information, please see http://msdn.microsoft.com/en-us/library/aa258278(SQL.80).aspx. 4. If the database cannot be repaired, the another option is to export data from SQL Server 2000 to SQL Server 2008.