Actually if you want to compare apples to apples you should consider comparing Windows XP to Gnome or KDE in terms of the graphical shell. If you are trying to compare the underlying operating system then you would want to compare the XP kernel to the linux kernel.
Try http://kernel.org/
You'll probably find that the Windows 98 kernel is more different from the XP one than XP is from linux I would expect. XP is a version of Windows NT (5.0 internally) and that was rebuilt from scratch using a team led by, and principally composed of, members of the old VMS team.
PS. @Linq Absolutely.
It's ironic that the success and longevity of the PC is almost entirely due to the abject failure of IBM to stop all the copying of it. Without the cheaper copies, the PC would certainly have died in the water.