Different CD Keys will make the install act differently. There's a CD key for Office XP, for instance, which will just allow the install and no worries, there you go. There are other keys, however, which will make the registration persistant, and will deactivate the product if you don't reg.
Not that I know, I heard this from friends.
Also note that certain CD keys for Win XP are not recognized by SP1. Meaning SP1 for WinXP won't install.
XP home and Pro will run like 90 days if you do not register it with MS. You can only register it once, so you can't install it on 2 machines and have it keep running.
The XP pro with a corporate license doesnt have this restriction, you can install it as many times as you want and never register it. Some XP PRo corp keys are banned by MS where you can't install SP1.
There may be ways around the XP home and PRo non corp registration, I just never tried.