If you want to find the CD key, click Start > Run > type regedit.

Browse to the key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion
and look for ProductID. It should be the of the form XXXX-XXXX-XXXX-XXXX-XXXX.

Installing a downloaded XP with a legit CD key won't give you any trouble with updates or any kind of Microsoft validation. As far as they are concerned, it's a legal copy of Windows, since the key is paid for.