NIC's are one of the items that are checked though. (modems aren't)

From the READ1ST.TXT:

Products: Windows XP Home Edition, Windows XP Professional only

If you are prompted to activate your installation of Windows XP, defer
activation until you have completed planned upgrades or changes to
your computer's FIXED hardware and their device drivers. Otherwise,
you may need to reactivate after making such changes. FIXED
hardware/drivers include the first instance of devices such as
video, network (excluding modems), SCSI, hard drives, CD-ROM/DVD, and
system memory. USB or other removable devices (such as cameras and
printers) do not affect activation.