Hi Steve,
Can you please follow the steps below and let us know if you are successful?
1. Uninstall previous versions of FXLC95000 Mailbox tool if any. Also uninstall previous versions of ISF 1.1.
2. Run ISf1.1 installation.
3. Select "Typical" installation and deselect "Tools" and proceed with installation.
Check if you can complete the installation successfully for ISF 1.1 except Mailbox Tool which is available under Tools section. Please let us know your results.
Also please let us know your PC configuration. Is it 32 bit? Windows 7? Processor?

Thanks
Aswin