Ignoring your troubleshooting already completed, as it appears you've already done most of it, it sounds like:
-something still exists in C:\Nexon // solution: recheck and delete
-the file you are downloading is corrupted (Nexon server issue) // solution: keep trying until it works
-the download is being interrupted on your end of things, and the installation is trying to commence from an incomplete download // solution: check your firewall/AV/modem connectivity/try using an ethernet cable (instead of wifi). If the download ETA is 1 hour, and it begins installing after 30 minutes, then it's probably your connection somewhere.
-the file is corrupted because you have a bad sector on your drive, or you don't have enough empty space on your drive // solution: get a new drive
edit: it could also be a W8 issue. You could try checking to see if any system updates affect it by rolling the system back to the original install date.