Broken mopac-win
There is a Windows-only windowed version of MOPAC, mopac-win, that opens a window displaying a summary of a MOPAC calculation in progress and allows calculations to be paused. This functionality was implemented with Intel's QuickWin library, and does not allow for an I/O channel to be shared between the QuickWin main application and a dynamically linked library. Since MOPAC is presently compiled as a DLL, it is not able to display its output in the window as intended. Presently, mopac-win is omitted from the Windows distribution of MOPAC.
There are two possible solutions to this problem. A simple, inelegant solution would be to statically compile the core MOPAC library into the mopac-win executable, which will roughly double the size of the MOPAC Windows installer. A more complicated solution, which might allow MOPAC to be integrated into more GUIs in a more flexible way, would be to set up a simple callback interface that shares output strings explicitly rather than through an I/O channel.
Unless there is appreciable interest in mopac-win, this issue has a very low priority.