Under U.S. Code Title 17, Section 108, libraries and archives may reproduce copyrighted works for preservation, but only if the work cannot be obtained at a fair price. Since Microsoft no longer sells Windows XP, one could argue for fair use. However, no court has explicitly validated abandonware archives.
This is the gray area of abandonware. Let us break it down:
Not all Windows XP ISOs are created equal. WinWorld hosts a diverse library. Here is what you will likely encounter when you visit the page: