Last night I did some testing and installation of Sql Server 2005 and Sql Server 2000. I installed the RTM version of Sql Server 2000 as the default instance first. I then installed RTM version of Sql Server 2005. Afterwards, I noticed that I couldn’t connect to my Sql Server 2000 instance any more. I am running Windows XP SP2 on my laptop. I had to manually start the default instance.
So keep this in mind when you are doing testing and installation, especially in production environment.