hello
if you remove packer from the question, will be a normal windows question on how to do a silent install.
all this is answered in microsoft windows documentation.
you can pass unattended.xml over a floppy, or add it in some other way, all this is documented in Microsoft.
When you get familiar with this, will be easier to see how packer can help.
packer can create a virtual floppy with scripts and unattended.xml, that the Windows installer can read and use.
my suggestion will be get familiar with windows silent install first, do some custom installs without packer.
when that is done, find some packer project on any provider, ie Virtualbox, see now its done, and then all will be more clear.
from there, create your own (based on a working existing one) will be easier.
alvaro