I don't remember the specifics, but this is one of those many things that require you to set up a WINNT.SIF file on a floppy disk. If you google up winnt.sif you should be able to find more detail.
Microsoft dumbed down the installer to the point where you can't really control anything without WINNT.SIF. This is also how you accomplish changing the path of things like user profiles, or the "Program Files" directory, etc.