Question : MS SQL 2005 and MS SQL 2008 on one box?

Can MS SQL 2005 and MS SQL 2008 co-exist on one box?  If not, why?  If so, are there anything unique in the installation or configuration?  Ports, processes?  

Answer : MS SQL 2005 and MS SQL 2008 on one box?

Yes you can have SQL Server 2005 and SQL Server 2008 on the same server.

One will need to be the default instance, and one will need to be a named instance.

Default = <servername>

Named Instance = <servername>\<instancename>

Each instance is installed in its own installation directory and has its own port number, 1433 is the port number of the default instance and the named instances will have a random port number assigned at installation.

See the following website for more info on SQL Server instances.

Instance Configuration
http://msdn.microsoft.com/en-us/library/ms143531.aspx

Typically you would go with a SQL Server 2005 default instance, and a SQL Server 2008 named instance.