I'm writing something in .Net that automates Excel. That's generally fine
but I have an issue with a user who has an add-in installed such that
whenever they go into Excel, it pops up a login screen. Is there a way of
replacing...

objExcel=New Excel.Application

with

objExcel=New Excel.ApplicationWithNoAddIns

I know that I could probably do something like find the registry key that
tells Excel about the addin, turn that off, run Excel, and then switch it
back on - but that seems a bit kludgy. Is there a better way?