One of the steps of upgrading from 2000 to anything later (2005, 2008, 2008R2, 2012) is to update all statistics. That is a post-upgrade requirement.
If you perform that step - it should take care of these types of errors.
The next item you want to perform is: DBCC CHECKDB(database) WITH DATA_PURITY;
This is a one time operation and should complete successfully. If it doesn't complete successfully, fix the errors either on the 2000 instance (because someone manually modified the system tables) or on the 2005 instance by recreating the affected objects.
Jeffrey Williams
“We are all faced with a series of great opportunities brilliantly disguised as impossible situations.”
― Charles R. Swindoll
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Managing Transaction Logs