Instance to Upgrade
Use the Instance to Upgrade page of the SQL Server Installation Wizard to specify the instance of SQL Server 2000 or SQL Server 2005 that you would like to upgrade to SQL Server 2008.
Use the radio buttons to specify whether to upgrade the instance of SQL Server and its shared components, or to upgrade only the shared components. Shared components in this SQL Server release are the following:
SQL Server Browser
SQL Server Active Directory Helper
For information about supported upgrade scenarios and upgrade known issues, see Version and Edition Upgrades.