I'm not from Laminar Research, just a fellow simmer in the UK, but probably the best thing to do is make certain that none of the original (DVD) X-Plane 11 root directory is on your PC, then download the 'demo' version from LR's website.
You should then be able to install and load the demo copy of X-Plane 11 onto your PC, but you will have to make sure that DVD disk No 1 is in your DVD ROM tray at all times, as this will be used to activate the demo copy.
I understand that you should be able download updates from that point, but again, your DVD Disk No 1 must be in the DVD ROM tray throughout.
The whole X-Plane 11 root directory is completely independent from any program files on your PC, so it would be best to place the root directory as follows C:\X-Plane 11.
X-Plane could put all folders and files on the Desktop, as it did with me once, but I understand that you should be able to change the location to your C:Drive during the initial stages of the digital download install.
Hope this helps and I am assuming at this stage that your copy of X-Plane hasn't come from Steam?