It seems as though winter has set in early this year and we may have been caught off guard. Over the past few years, winters here in the Lehigh Valley have been rather easy on us. We must, therefore, make sure we have winterized our horses and barn before the true winter months of January and February set in.
We should start with first making sure we have an ample supply of hay on hand for the horses. The last thing you want to do when a storm is expected to roll in, is scramble to get more hay into the barn. Your local farmer may not have any more left, at which point you will need to find someone that does have hay. If you are lucky enough to find someone, he may charge you an arm and a leg for it because of a low supply and a large demand. Make sure you have brought in enough hay for your horses to last throughout the winter. If you are short on storage space, purchase the amount you will need and see if the farmer is willing to store it for you until you need it.
The next thing you should do is make sure the water pipes are well insulated. When a deep freeze hits, you do not want to end up with frozen pipes and no water. If your pipes freeze, they may burst, which means major problems. Ever try lugging buckets of water from the house to the barn? Lots of work and no fun, not to mention freezing cold. Heat tapes are a good idea to place on the pipe under the insulation.
Place water heaters in the horses buckets and water trough. It's a lot easier than trying to chop the ice out of the buckets every day. Plus, the horses prefer drinking warmer water rather than ice water when it is cold out. Would you rather drink hot chocolate or iced tea when you are cold?
Make sure the horse blankets, if you use them, are clean and in good condition. The blankets should have been cleaned over the summer months. If the blanket needs repairs, get them done, IMMEDIATELY. Don't wait until the freeze sets in. Some horses grow thick, heavy coats over the winter and don't need to be blanketed. Make sure these horses have no mud stuck to them. They need to fluff up their coats to keep warm. If the coat is packed down with mud, they are unable to do that and will freeze. If they are covered with snow, don't worry about it. Believe it or not, snow is an insulator. Just don't brush the horses when they are wet. This will make the coats lay flat next to the skin and give them the chills.
Draft proof your barn. If there are drafts in the barn, take care of them. If the horses have to stand in a draft, they will get ill. You don't want to call the vet out in the middle of a snow or ice storm because of your stupidity. Good way to make enemies with your vet. The barn should not be air tight, either. Air should be able to circulate throughout. 
Get the horses hooves taken care of. You may want to pull the shoes if you will not be doing much riding. If you must shoe your horse, a good idea would be to place pads between the sole and the shoe. This will prevent the snow from balling in the foot. If you have ever seen snow ball up in a hoof, it looks like the horse is walking on high heels. The other option to putting pads on is make sure you have lots of cooking spray on hand. Spray the horses hoof before letting him out into the snow. It's not as effective as pads because it does wear off eventually, but it's an alternative. Also, the shoes should have borium on them or you should be able to place studs into them. This helps the horse have traction on the snow and ice. Plain shoes tend to be slippery.
Horses need plenty of water to drink. Horses drink lots of water during the winter months, also. This helps the food slide through their digestive tract. If they are unable to access enough water, they may end up with impaction colic. As many of you know, colic can be fatal to horses. Heated water is more inviting to drink than ice water. There are more cases of colic in the winter months than at any other time of the year. This is all because the horse's water supply is frozen and he is unable to drink.
Check your fences regularly to be sure that no damage has been done.
Last, but not least, the horses should have some sort of shelter when they are left outdoors. The horses need the shelter to get out of the wind and the elements, when needed.
Your can now sit back and watch the horses play in the snow. You might even catch them making horse angels in the snow. Enjoy your horses, even if it is winter. Winter doesn't have to be a dreaded season.
And to think.... I thought that all you had to do was add antifreeze to the horses!!!!
Seriously though.... good advice. I don't have horses (just a cat, dog and kids) but reading through this.... it all makes great sense and is well written.
(Love the Lehigh Valley!!!! My old stomping ground as a kid!)