Hi there. That's quite a tricky sounding situation. My first suggestion would be to try running as admin but you have already done that. I think your intuition may be correct concerning some kind of antivirus blocking it from accessing the hard-drive. Conversely, it could actually be an infection on that machine that is causing you trouble - something that is trying to prevent anti-virus from detecting itself by dictating what programs running on the machine are able to read/write. The folder that PixelCNC outputs log files to (or is supposed to) is one that it should have access to even if you are not running as administrator - it's a special area that Windows permits as being a safe place for programs to write to the hard-disk without the threat of them corrupting the system or anything like that. Since it's not able to read or write anything from disk I would suspect that it's something that could be running in the background preventing PixelCNC from accessing anything.
I would suggest checking what programs you have running - likely some things are sitting right in your system tray on the right side of your task bar - you could try investigating those. It very well could be some kind of anti-virus that's purposefully preventing PixelCNC from interacting with the disk drive because it's currently an unsigned executable - which is to say that it has no identifying information proving where or who it originated from. This is because many viruses are also unsigned - authors must use government issued identifying documents to prove their identity to a central authority that distributes certificates to publishers and developers and someone writing malware does not want to jump through those hoops only to tie their real identity to their malware. I simply have not jumped through the necessary hoops yet to sign PixelCNC but will do so once it reaches beta in just a few more months. This may be causing overzealous anti-virus to completely block it entirely. Ironically, malware authors have since begun stealing code-signing certificates to sneak their malware right under the nose of anti-virus, so it's become somewhat futile - but I suppose it does prevent low-effort malware from wreaking havoc on a system.
Try temporarily disabling any AV you have running - or going into its settings and seeing if you can manually add PixelCNC to some kind of exclusion white list. I would think there is something running on there that isn't running on your other two systems, and maybe that would be a good clue as to where to start looking.
Let me know if you make any headway or find any clues. I'll check in with some of my software development circles and see what I can find out.
Charlie