Short Answer:
After installing UBB, I gave IIS_IUSRS (MYCOMPUTER\IIS_IUSRS) write permission on all the directories UBB flags as needing write permission.
This didn't work.
After much trial and error, I found that you need to add the IUSR user (different than the IIS_IUSRS shown by default) and give it write permission on all the directories UBB flags as needing write permission.
Notes: I reverted IIS_IUSRS to the default 'read & execute', 'list folder' & 'read'.
Then added IUSR and gave it 'read & execute', 'list folder', 'read' & write.
All is working flawlessly.
I can log into my messageboard both locally via localhost and remotely via IP address.
PS. You do not need to download and install PHP and MySQL via external sources. You can use Microsoft Web Platform Installer. This is a wonderful tool that installs these components, handles PATH requirements, and any .ini config. After installing these components via Microsoft WPI, I was ready to go and all defaults were sufficient to run UBB.Threads.
