I have a server with a default instance and 2 named instances of SQL Server 2005 standard installed. This is a mission critical production server that cannot be restarted during normal business hours.
Will uninstalling the two named instances of SQL Server 2005 require a reboot or put the server in a state that may cause issues with the default instance of SQL Server 2005 until it's rebooted?
This would probably get a better answer at serverfault.com.
I'm not sure how much it would help perf, if SQL isn't getting hit it doesn't do much. You could probably get away with uninstalling, but then again when in surgery bad things happen. I've never killed SQL server uninstalling an instance, but I have killed the client tools. I would take one of the following approaches:
a) First, backup and drop all the databases to reclaim the disk space. Then stop disable the services for the named instances. The binaries will still be there, but they aren't too large and will be sitting idle.
b) Better long-term plan if you can source the hardware is to setup a new de novo box and drop a clean SQL instance over there, then port the live server over. Really not too painful. Then repurpose old box as is fit.