Windows XP files missing for internet connection

I have installed a cable modem and router, but the
installation company (Adelphia) says that files are
missing from my computer that allow their modem to talk
to my computer. Can anyone tell me what files are needed
and how I can reinstall them? We tried hooking up my
son's computer which was purchased at the same time and
it worked perfectly. I am hoping for a solution that
does not require reformatting the drive and reinstalling
XP. Any info would sure be appreciated.