The Windows Ease of Access Center (find it in the Control Panel) provides a way to make the keyboard focus follow the mouse pointer. Click the "Make the mouse easier to use" link and then check the "Activate a window by hovering over it with the mouse" checkbox.
This can get you part way there, but if you enable X-mouse style behavior this way you get the unfortunate side effect that when a window gets focus, it also comes to the front.
To get the true X-mouse behavior requires a registry tweak. The registry key of interest is HKEY_CURRENT_USER\Control Panel\Desktop.
- Edit the UserPreferencesMask value, changing the first two bytes to 9F 3E.
- Create a new REG_DWORD item named ActiveWndTrkTimeout. Its value is the time delay in milliseconds after the mouse pointer enters a window before it receives focus. In my demo video I have the value set to 200 decimal. Set the value to whatever works best for you. A value of zero will cause you to have trouble with pop-up and drop-down controls like drop-down lists and calendar date selectors.
- Log out or reboot your computer for the changes to take effect. Logging out might be sufficient but I'm not sure about this.