After changing the battery and the starter my 1999 ford windstar engine will not turn over

Are there any warning lights on the dashboard? The next obvious thing to check is the battery. A starter needs a lot of voltage to turn it. Vehicle batteries are considered 'Old' when they are more than 5 years old. If you know how to use a voltmeter or a multimeter you can test the battery.
With the ignition key Out the battery voltage should be somewhere between 13.6 and 14.2 volts. Get a helper to crank the engine and watch the voltage reading. If it drops below 10 volts the battery could be faulty.

Won't start how ? When you turn the key to start does the starter engage the engine an spin the engine ? Crank - no - start or a no crank - no start ?

How can a starter run if the battery is disconnected, please explain.

Is gear stick in park, try starting by earthing starter, then you can trace fault, key start might be problem

is it getting spark , if not then check the fuel system then the starter .it might even be something as simple as your starter relay.

try replacing the starter motor. can also be the connectors to the battery, starter, or the groundwire connection at the battery or engine block. clean all connections and use dielectric grease on all connections

2003 Ford Truck Windstar 3.8L SFI 6cyl Repair Guide

Ford Windstar 1999-2003 - Starter Removal & Installation

1. Disconnect the negative battery cable.
2. Raise and support the vehicle safely.
   NOTE When removing the hard shell connector at terminal -S-, grasp the plastic shell. Do not pull on the wire.
   
3. Disconnect the starter electrical harness.
4. Remove the upper starter bolt.
5. Support the starter and remove the lower bolt.
6. Remove the starter from the vehicle.
   
   
7. 
   Fig. The starter is located at the lower front of the engine, and is visible through the air duct
   

To install:

1. Position the starter in the vehicle.
2. Install the upper and lower bolts. Tighten to 15-20 ft. lbs. (20-27 Nm).
3. Connect the starter electrical harness. Tighten the starter cable nut to 80-124 inch lbs. (9-14 Nm).
   NOTE When installing the hard shell connector, be careful to push it straight on and make sure it locks in position with a notable click or detent.
   
4. Lower the vehicle.
5. Connect the negative battery cable.

I really hope helps with this (remember rated this help) Good luck.

it may be your solenoid which is connected to your starter. You should also check the wires going out of your starter. They could also be frayed.

have you tried disconnecting the battery for 15 -- 30 minutes, it may allow the anti-theft device to reset....

You will have to look at other areas, like spark and fuel, and engine compression.