"Multiple Instances of this application's Installer are running. This one will stop and let another complete."
FIX BELOW:

This had popped up a few times when I have tried starting up SK and was getting to be pretty frustrating. There were no visible running processes that seemed like it could be the problem when I went looking the first time. I looked around online and in the forums to see if anyone also had this problem, or if there were any fixes and didn't find any.

Anyway, I figured out the solution on my own and thought I would share it here. I am sorry if this has already been answered, but I didn't find it, and wanted to share the solution.

When the pop up saying: "Multiple Instances of this application's Installer are running. This one will stop and let another complete." comes up, open task manager. Right-click the "Spiral Knights " application and press "Go to process" there is a process running called "javaw.exe*32" then press "End Process"

That will not be the only process called "javaw.exe*32" be sure to close all of those named processes in the process list. You can do this by highlighting one process and pressing the "J" key to find all of the other processes that start with that letter. After they have all been closed you should be able to launch the game normally again. I hope that helps.