The hard drive has two partitions. I'm installing Windows 7 on the second one. It automatically gets assigned the drive letter C (and the first partition becomes D). Is there any way to override this assignment during installation? It's a dual-boot system, and I want drive letters to be consistent. On the vanilla drive selection dialog, there's no letter assignment UI.
You may not be able to change it in setup but after installing you may be able to change it by going to Control Panel > Administrative Tools > Computer Management > Disk management and right clicking on the drives. There's an option to change the drive letter there, however i'm not sure if that will screw anything up. Try it and find out? Good luck!
You need to initiate the custom installation (setup.exe) after booting into a compatible version of Windows. That way the drive letters are already assigned to the partitions, and Windows 7 installation will assign the same drive letter for the chosen partition. From my testing, XP64 is able to start the x64 setup, and it should work for Vista x64 as well.
Note that some of the later (made in 2015+) installation media are compiled with new libraries, making them incompatible with XP, but they can still be used (without booting into XP), just not for the purpose of this question.
There is a workaround for this. You can install Windows 7 first, on the first partition, then install the other OS on the second partition, then restore the bootloader.
Another way would be to change the letters 'offline'. I.e.: use a bootdisk to write the preference of drive letters to the hard disk. There should be software capable of doing this.