It seems this is a compatibility issue with Windows 8. Once I set the Python command line to execute with windows XP compatibility mode the comtypes module worked as expected.
This same fix also applies to the win32com module, which I tried but experienced similar difficulties with.
To set compatibility modes in windows 8, you can follow these steps:
- Go to your control panel.
- Click on the 'Programs' category.
- Click on 'Run programs made for previous versions of windows'
- Step through the troubleshooter, selecting the Python command line as the target program.