InspIRCd packages are available on the InspIRCd download page.
First, download the installer from the download page (linked above).
Once you have downloaded the installer, execute it and follow the instructions on screen.
Once it has finished installing, it will be available in
C:\Program Files\InspIRCd on 32-bit
C:\Program Files (x86)\InspIRCd on 64-bit Windows.