Found the answer, it was right in front of me but I refused to read it....


To set INSTALLDIR to point to a root drive, follow these steps:

Select the General Information view.

Click Product Properties to display the property grid.

Select the INSTALLDIR property in the right panel.

Click the ellipsis in the value field for this property and select the [WindowsVolume] option in the browse dialog that appears.
This folder places the installation on the root directory of the target machine because the property [WindowsVolume] evaluates to the drive letter that contains the Windows operating system.