The Swamp!

Installing Windows XP is not a difficult tasks, However there are a few things that needs to be taken into consideration here:

1. If you are installing on a current installation, make sure to backup your stuff first, That is all your files and documents and personal data that you saved on the computer.  Programs cannot be saved as they require certain registry and system parameters that are installed during one program's installation.
2. If this is a fresh installation, make sure your PC can take it.  I have seen Pentium 3's with 256mb of RAM do the job fine, but something new and nice would go better :)
3. Make sure you have the product key and all that mumbo-jumbo.

Sometimes you have the need to change your windows XP Serial number, wether because you had an illegal copy and bought a license, or you are ghost-installing a bounch of similar computers and you want to install each's individual key.

There are two methods I know of for doing this:

Method 1: Microsoft's Official Tool

Microsoft's Official Windows Product Key Update Tool (for changing keys only):
http://go.microsoft.com/fwlink/?LinkId=50346&clcid=0x409