My serpintine belt on 2000 siloette keeps shredding

Sounds like one of the pulleys is not aligned with the rest. You can use a straight edge to check the alignment.
I would also want to make sure all of the pulleys are secure and not loose or bent.

You need a trained tech to listen to this noise, see the local dealer.

Yes that is very likely it. Since you changed the belt, you should know - Is it the tensioner pulley ? Otherwise it is probably the water pump. The problem is the bearings.. If the bearings are locking up, the resistance will shred the belt quickly.. the part needs to be replaced is the bearings. Waterpump bearings are built in to pump. Change whole thing. (may need to transfer the pulley)

http://www.ehow.com/video_2327530_reinstalling-serpentine-belt-air-conditioning.html

i wold pay particular attention to the age and springiness of the tensioner. if the tensioner spring, not just the pulley, gets old it will not keep adequate tension on the belt, it will get too much slop in it and jump ship.

If it's seized then yes, you should check all the other items that run by the belt.

Typically a belt will come off if all the pulleys are not perfectly aligned.
If you recently replaced a power steering pump and the pulley was not properly seated all the way down where the pulley front is flush with the metal shaft from the pump then you will shred the belts and they will come off. If you ONLY swapped the tensioner and idler pulley you may have installed the tensioner so it isn't parallel to the other pulleys but that would be very rare because it would have been difficult to get the belt on if it was not properly installed. There also could be a damaged plastic pulley that could be "cutting" into the belt which would also cause the belt to slip off.

There are 2 possible engines for your Windstar, the 3.0L and the 3.8L. Below are both diagrams. Best of luck on your repair!
Greg

3.0L






3.8L

PULLEY ALIGNMENT.WITH BELT ON LOOK AT THE PULLEYS FROM THE SIDE.U MUST HAVE A PULLEY THE IS MISSALIGNED

With the belt off use your fingers to run the entire circumference of both pulleys. Check for any burrs or sharp edges. If it is all smooth proceed to pull on each pulley face to see if you can rock the pulley up or down or side to side (we're looking for play in the pulley, not spinning)

Also make sure that your tensioner is adjusted correctly,

Possibly who ever replaced the water pump did not line up one of the pulleys or an idler correctly. If it is not spaced correctly the serpentine belt will not follow the grooves in the pulley. This will cause the belt to jump out of the grooves and jump track and rub on the pulley and shred it within a few minutes or seconds depending on how badly it is misaligned.Check the alignment on the pulleys and the routing path of the belt. Both must be correct in order to work properly.