Hi XanderPaul
First make sure to unhide hidden files and folder, and that you can see the filetype name of known files.
You can see in below link how to do that.
http://www.bleepingcomputer.com/tutorials/tutorial151.html
Next is finding the folder {C917BA70-28A3-4C74-B163-41FD8C8E1A5A} in the Installshield Installation Information folder and delete it. Before deleting the folder make sure you found the correct folder by opening and then checking the contents of the file Setup.ini if it contains a line saying AppName=Stronghold (if you delete the wrong folder you will have a program that can not be uninstalled), if Setup.ini does not contain that line you will have to search the other folders with strange names until you find the correct one.
Make sure to delete the contents of the Temp folders thereby making sure there are no remnants of the game left there from the faulty installation. Also manually check in the Program Files(x86) folder if there are any remnants of the game there, if there is delete it.
Now try again instaling the game.