Any ideas? Worked fine last month, now this.

I deleted everything, redownloaded it and still I'm getting this.

How about a screenshot of the directory with all the installer files, including showing the file extensions, and the specific error?

Oh right. :P

Hold on.
Will get that posted later today, I hope. I think this covers it.
You see the root in the back, I put all the downloads into a temp folder named SHADOW. Then you have the installer asking for the file, being told it is in SHADOW and then the error telling me it can't find it.

http://imgur.com/a/JyNvV

Except for showing the complete filenames. ;) You have plenty of screen real estate. Drag the Name column separator to the right and it will display the full name. Based on the Type column files have the correct extension.
The "file can't be located" error will happen if the file is missing, something renamed it (missing), partial/incomplete download (hash mismatch), corrupt download (hash mismatch), and wrong version of the bin for the given setup.exe (hash mismatch).
As it's asking for part 1, the larger 4 gig .bin file it's crying about. Your setup for installing it is good, but given you have now downloaded it a couple of times double check the files don't have any extra (1) or the like added to the filename. If the name is correct than the problem is a file hash mismatch. Ideally you would check the hash (MD5/SHA1) of the -1.bin file you downloaded and compare with someone else's hash of their download for that file. If they match & you did a fresh download of the setup.exe then the problem is the wrong version on the server. Someone need to tell GOG so they will go and fix it.
I would give you the file hash myself, but I only have the demo.

When my bandwidth resets next week I'm going to try downloading from the old downloader which is fewer files and see what happens.

It's less than three files? The first two are patches which aren't relevant to running the full installer. Otherwise, I suppose. You might want to ask about this in the "what did just break" thread, and see if someone can check if their file is the same. Which would save you from wasting the 4 gig or so of bandwidth if it's actually a GOG problem. Though that would entail the full, complete filenames being displayed.

Yeah, the GOG Downloader is one file.