At what stage you are getting this message? While installing or running the tool?
With which version you are getting this message? With 9.1 or 16.3?
Generally such messages are for informing users about file extension association changes and things should work either way and according to selection made by you.
In reply to alokt:
In reply to dyoswa:
For 16.3: You can use any one of the following:
#1 It apprears that you have UAC turned on your system. Please turn off UAC (bring the slider to lowest level) and reboot the machine (as this change would be effective only after reboot). Now try installing 16.3 again. It should work. UAC controls are available under Control Panel\All Control Panel Items\User Accounts
#2: Download the latest ISR and install it. It should also fix the issue of missing PSpice toolbar.