The Certification Authority registration and security settings in Active Directory must be upgraded from the Windows 2000 ...

The Certification Authority registration and security settings in Active Directory must be upgraded from the Windows 2000 format to prevent tampering by unauthorized individuals.  This will not cause interoperability issues with clients and should be done as soon as possible.  If you do not choose to upgrade the settings now, you will be prompted again the next time you open the management console.