1. Copy the whole dvd to a directory on your harddrives
2. Download an msi-editor, for example Orca
3. Edit the msi with orca
4. Search for "Dx", "DirectX" and "Drv" and remove ALL lines found by orca
5. Remove the table LaunchConditions, because it prohibites starting the installation under Vista
6. Save the new .msi and quit orca.
7. Now you have to do one thing more, and i don't know if i can post it here. In Germany, where i come from, it is disallowed, so i don't write it here, too.
Hint: We have removed the check in the installer for the "DrvSetup" and "DrvSetup_x64". When you know for what the files stand for, i think you propably know what you have to do next.
And for all of you: Here is the msi file for Fahrenheit, if you have Indigo Prophecy try the steps described upside.