POSSIBLE FIX
Eagle51... I'm sure this reply is way late but I'm hoping it may be helpful to someone else like me who found your question while searching the exact error as you experienced.
1) I had changed the install location to C:\Xplane (instead of the Desktop.)
2) While installing files from Disk 3, it failed and complained about permissions.
*Note: I'm an old hack for a long time and took a lazy stab at it by changing the Xplane folder permissions to allow EVERYONE FULL CONTROL. After that, it pushed right through to the end. No burps no hiccups. :)
I'll change the permissions back to original after I get everything setup the way I want. It won't really be necessary since this computer never touches the internet and I'm the only one who knows. LOL.