How to use Ctrl+left/Ctrl+Right (instead of Alt+left/Alt+right) to move to the previous/next word
You can use KeyRemap4MacBook with a private.xml such as this:

&lt;?xml version="1.0"?&gt;
&lt;root&gt;
  &lt;item&gt;
    &lt;name&gt;⌃← &amp; ⌃→ to ⌥← &amp; ⌥→&lt;/name&gt;
    &lt;identifier&gt;remapCtrlLeftAndCtrlRightToAltLeftAndAltRight&lt;/identifier&gt;
    &lt;autogen&gt;
      __KeyToKey__
      KeyCode::CURSOR_LEFT, VK_CONTROL | ModifierFlag::NONE,
      KeyCode::CURSOR_LEFT, ModifierFlag::OPTION_L
    &lt;/autogen&gt;
    &lt;autogen&gt;
      __KeyToKey__
      KeyCode::CURSOR_RIGHT, VK_CONTROL | ModifierFlag::NONE,
      KeyCode::CURSOR_RIGHT, ModifierFlag::OPTION_L
    &lt;/autogen&gt;
  &lt;/item&gt;
&lt;/root&gt;


I'm not aware of any System Preferences setting regarding this.



With regard to comment, change the contents of the &lt;autogen> to…

__KeyToKey__
KeyCode::CURSOR_LEFT, VK_OPTION | ModifierFlag::NONE,
KeyCode::CURSOR_LEFT, ModifierFlag::CONTROL_L




__KeyToKey__
KeyCode::CURSOR_RIGHT, VK_OPTION | ModifierFlag::NONE,
KeyCode::CURSOR_RIGHT, ModifierFlag::CONTROL_L