Well I was able to fix this problem by completing the missing details of each user directly in the database tables... This problem was not created by DNN... maybe a third party application is the culprit... but if someone comes across this issue maybe this will help.
This involves working with the SQL database directly so if you don't know how to use the enterprise manager you should probably not mess with the database as you can cause some major damage to your DNN Install.
Also I am using SQL 2000 so it will be different for people using SQL 2005
Open the Enterprise Manager and browse to
Console Root > Micrososft SQL Server > SQL Server Group > SQL Server Instance > DNN Database > Tables
Here Open the aspnet_Membership, aspnet_Users and Users tables using "Return all Rows" and fill in any missing information for users that have blank columns. Mostly this will be information like Email, LoweredEmail, FirstName, LastName etc.
Once done exit the Enterprise Manager and then when you browse to the Users Accounts module from the Administrator Panel > Common Tasks > Users and click on "All" and a user list is presented and the error dissapears.
AGM