Windows XP still ships with a subsystem called POSIX, which allows the use of Unix commands. Disabling POSIX prevents hackers from using Unix commands against your system. Go to Run and type regedt32 (not regedit). Find

HKEY_ local_machine\system\currentcontrolset\Control\Ses sion Manager\SubSystems

and click on the multistring called Optional in the right-hand pane. By default, the multistring's value will be POSIX; delete that value and leave the space empty (but don't delete the Optional multistring). Then click on the actual POSIX multistring in the same pane. Note that it points to a file in your Windows System32 directory called Psxss.exe. Delete that file using Windows Explorer, use the Registry Editor to delete the POSIX string, and then reboot.