You can find XP Pro sp3 embedded in "XP Mode." See, e.g., https://zeltser.com/how-to-get-a-windows-xp-mode-virtual-machine-on-windows/ Note that when XP Mode is installed in Windows 7 Pro, as intended by Microsoft, it includes a license key and is automatically activated. It's not clear from these various "how
to" guides, but it's my understanding that the embedded license key will not work when you extract the virtual hard disk from the XP Mode download. A valid retail license key probably will work, although you may have to activate by phone.
Other than XP Mode, Microsoft has never made an installer file for Windows XP available for download. When you see links to "XP Professional Service Pack 3," those
WERE links to the file necessary to install the service pack for an already-existing installation of Windows XP Professional. However, AFAIK, none of the links to Service Packs for Windows XP are still valid.
The service packs themselves (SP 2 and SP 3) still can be downloaded from the Microsoft Update Catalog. These service packs can
NOT be used to install Windows XP.