Anyone still running Windows XP won't be able to argue they weren't warned that support is stopping soon. From later this week, machines running XP will start displaying pop-up notifications that support and patches for the operating system ends in April.
From 8 March, Windows XP Home or Professional users with updates switched on will see a desktop notification reminding them of the looming end of support. While it's possible that some rusted-on Windows XP users don't have updates switched on, you'd expect most users to encounter this message. Microsoft is also offering free copies of a new data migration tool, PCmover Express for Windows XP, to help individual users migrate their data when they upgrade.
Windows XP support ends on 8 April, though Microsoft will continue to produce security signatures for its own security software for 12 months after that and will provide expensive paid support for corporates who haven't yet switched onto a newer version.
New Windows XP data transfer tool and end of support notifications [Windows Experience Blog]