If you cannot connect, you should be able to call Microsoft and get it over the phone without any problems.
The purpose of the activation key is keep people from using Windows XP without paying for it. So the only way you can get a legitamate activation key is to give Microsoft a legitimate CD-key.
You will only need one, providing your system stays about the same. There are no time limits that I'm aware of... Changing your hardware configuration significantly for Windows XP Home edition will cause Windows XP to ask you for a new activation key, however. I think Windows XP Professional is supposed to be a little more lenient on requiring new keys.