First of all, this trick must only be used by those who have legal copy of XP and are troubled by registrations reminders every now and then. Before you begin, make sure PC must not be connected to the network. During XP setup answer no when setup asks you to use Windows Dynamic Update.
Dynamic update connects to Microsoft to register XP. Once the installation is completed, reboot to Safe mode.
Go to Windows Explorer and rename %systemroot%\\System32\\regwizc.dll and
%systemroot%\\System32\\Dllcach\\regwizc.dll to regwizc.dll.bak or whatever. By renaming the
regwizc DLL, it blocks other windows components from registering XP to Microsoft.
Now, navigate to [HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\WindowsNT\\CurrentVersion] and look for the data key "RegDone" and set its value to 1 to make XP believe that its registered . Change the IE home page to something besides Microsoft's web site.
Reboot to Normal mode and reconnect your nic card before the XP Windows GUI loads.
