A. Setupdd.sys is included on the Service Pack 4/5 CD and in the Y2K download version of Service Pack 4 but not the normal version.

This file is needed to replace the one on the second Windows NT installation disk to repair a system that has Service Pack 3 or above. To create a set of NT installations disks insert the NT installation CD-ROM and type winnt32 /ox.

You can download SETUPDD.SYS here.