They say its a crash that it'll make you restart your PC, but in Windows XP you can ALT+TAB out of it. It doesn't seems to do nothing because it won't bring you to your Desktop, but its like you were there.
Then with key shortcuts, WIN key and pressing Up arrow twice, you can get to the Switch User option (without seeing it though, because Crysis should still be in front of your screen). After clicking on your same user profile all seems the same, but the Start bar should appear at the bottom now. That enables you to bring up Task Manager and kill the Crysis process.