I'm testing SQL Server database mirroring. I have two servers each with SQL Server 2005 installed as a 'default instance'. I've got the mirroring working without the 'witness' server. Now I need to test it with a 'witness' server. To do this, you need another instance of SQL Server. This can be on another server (but I haven't got another one spare) or a second instance on the same server. I know pretty much nothing about SQL Server so my question is how do I install a second instance of SQL Server on a server that already has SQL Server installed?