Open the install.bat file with a text editor and change the install path to your OCTGN folder. It's trying to install into the "users" folder which is a Windows 7+ location.
You would change:
set OCTGN=/My Documents/OCTGN
If that doesn't work change the path to:
set OCTGN=/My\ Documents/OCTGN
But I don't think you'll have to do that.
If in doubt, right click on your OCTGN folder>properties, and look at the folder path. Copy that into the "set" location.