First you will need the boot image file, you can download it from (right click save)
Next copy all the files from you XP cd to a new folder on you hard drive like C:\XP-CD\
Next, start Nero Burning ROM, and choose CD-ROM (Boot) in the New Compilation window. On the Boot tab, select Image file under Source of boot image data, and browse to the location of the Microsoft Corporation.img file. Also enable Expert Settings, choosing No Emulation, and changing the Number of loaded sectors to 4 (otherwise it won't boot!)
The ISO tab should be configured correctly as shown below:

If you have an older version of Nero you won't have the option Do Not Add ";1" ISO file version extention under Relax ISO Restrictions. You won't be able to boot your new CD, so update Nero!
You can configure the Label tab to your liking, I would however recommend that you keep the Volume Label the same as on your original Windows XP CD:
* Windows XP Professional: WXPCCP_EN
* Windows XP Home: WXHCCP_EN
* Windows XP Professional OEM: WXPOEM_EN
* Windows XP Home OEM: WXHOEM_EN
Next press New, and from the File Browser window, select the files and folders from your windows xp files location C:\XP-CD. Now drag & drop the selected files in the ISO1 window.
Last, burn your new CD.