Object name 'Clean Up History' in use when trying to complete the maintenance plan wizard in SQL Server 2008 R2
The job is "owned" by a maintenance plan in the following table, and I don't believe the names have to match. So you may want to check the following location:

SELECT name,id,description 
  FROM msdb..sysssispackages
  WHERE name = N'Clean Up History';


So you could rename that one, but it may cause problems later (some automated process may need to match on that name, or simply re-create it if it doesn't find it). So a much safer approach would be to use a different name for your own maintenance plan...