After XP installation comes Windows XP Product Activation (WPA), a step that was not included with previous Windows versions. Much ado has been made about this technology, but whatever personal feelings you might have, its intent is very understandable: to make it harder for people to steal Microsoft's software.
One note to put your mind at ease: activation and registration are not the same thing. Product activation is completely anonymous, and no personal information is divulged, as opposed to product registration, where you submit your name, address, contact information, and so on. You do not have to register XP. You do, however, have to activate it.
Here's how activation works: Windows Product Activation (WPA) associates a portion of the product key you enter at setup time with the system's hardware configuration. The hardware configuration is represented as a hardware hash, derived from a number of computer elements, including the video card, primary hard drive, CPU, RAM, network adapter, and CD-ROM drive (ignoring peripherals such as printers and digital cameras). WPA then combines these two valuesthe hardware hash and the Product IDto generate an installation ID.
The installation ID is sent to Microsoft. Microsoft WPA clearinghouse servers then validate the installation ID, and XP is good to go. Reactivation is only necessary under two circumstances: 1) when the operating system is completely reinstalled, in which case reactivation shouldn't be a problem, or 2) when the machine's hardware is substantially changed, in which case you might have to make a phone call to Redmond. What is a "substantial" change? According to Microsoft, a substantial change is when you upgrade four or more of the hardware components mentioned in the previous paragraph within 120 days of each other. If that's the case, you will probably be prompted to call the WPA clearinghouse and obtain a new activation code.
You have 30 days to activate, and trust me, you won't be given a chance to forget; pesky reminders display each time you log on. If you ignore these past 30 days, XP only allows a logon for purposes of activating. But because you have 30 days, you probably should wait a week or two before activating, allowing you to ensure that all hardware and software you plan to use is operational. This also lets you make configuration changes before activation without affecting the installation ID.
If this all seems a hassle, just consider that if you made and sold software for a living, you'd want to protect your products, too.