Today, I was using ServiceNow with on Windows XP with IE6 on a client laptop. I didn't have much choice as that was all I was allowed to use at the time.
Microsoft has ended support for Windows XP now, and many companies will "eventually" stop using it. I say "eventually", as I still see Windows 2000 and NT4.0 computers around.
XP was a great operating system, although somewhat insecure at times. I did switch to Windows 7 on my computers a few years ago, but even I haven't used Windows 8 yet. Maybe someday. :)
What does this have to do with ServiceNow? Not that much. ServiceNow does run on IE6, IE7, and IE8. However sometimes you get the "A script is running on this page" message and other client-side GUI issues. You are better off using ServiceNow on a modern browser, but you can make due with older browsers as well (try to skip IE6 though).