windows xp activating...and registering

There is an overview of activation of XP here.
http://www.microsoft.com/technet/treeview/default.asp?url=/technet/prodtechnol/winxppro/proddocs/WPA_overview.asp

You can activate your copy of XP online or over the phone with MS.

Remove any spaces from the above URL in order for it to work.

Just install your copy of XP as normal,
at the end of the installation process your system will automatically ask you to activate your copy of windows either now or later. If you choose now, it will also ask you to register (this is not a requirement but it is useful as microsoft will send you information) if you choose the option to register later you hare reminded everytime you boot xp to activate it otherwise after 30 days xp will lockout forcing you to activate it.

hope this helps