Installing SQL Server 2005 in the same directory as the current installation and replacing the SQL Server files
Installing a new instance of SQL Server 2005 and sharing the older databases between the new instance and the old instance
Installing a new instance of SQL Server 2005 on the same server as the old instance and moving the databases from the old instance to the new instance
Installing a new instance of SQL Server 2005 on a different server from the old instance and moving the databases from the old server to the new server