How do I keep my dog from barking at everything he sees in the window.

This is normal terrier behavior. They get excited when they see something moving. Your best hope is to first teach him the difference between barking and quiet. This is done by teaching your dog to "speak" on command. Once he understands the idea of "speak" it is easier to get him not to do it when you want him to be quiet. Here are some simple instructions on how to get started:
http://www.ehow.com/how_14237_train-dog-bark.html

There are also collars you can buy to stop barking. There is one described in this article called a citronella anti bark collar - this emits a scent that dogs don't like much better than a shock type collar.

Also other ways to train to not bark.

http://www.placervillevet.com/barking.htm

The other thing I was thinking - if he barks at everyone going by the window - why don't you keep him where he can't see out the window?

How socialized is the dog?

For example, my dog is poorly socialized with other dogs. He no longer barks at any humans (unless he's scared for other reasons) but any dog, cow, cat or anything he sees from the window he'll bark at them.

I have been training him, by correcting him each time he nuisance barks (I go to where he is, say "no bark" sternly, and a swat with a rolled paper if he keeps at it) but it hasn't eliminated the barking.

I think I will need to socialize him with other dogs (especially the neighbor dogs he most barks at) but haven't had a good chance to do so (no dog parks etc). What I'd do is walk them both side by side at a distance out of reach until they he's tired of barking and just ignores the dog. Hellifino if that would work.

And the last resort I'd try is a collar. From my research into it it doesn't sound like it's cruel (in some ways its less cruel than forever stressing your dog with discipline he's not fully understanding), but I'm not sure if it would be effective without the collar. In other words, the dog may just learn not to bark with that collar on, and that's not what I want. I don't want it to be a permanent thing.

Terriers like to bark. We had one of those once. The vet told us to put some
pennies in an empty soda can and shake it when the dog barks. The noise will
startle him and eventually he'll stop barking at every moving thing.

Pacco tended to warn me about wolves outside - but as my computer room was in the back and he was busy guarding me, this usually wasn't a problem.

It was at work, thus we built a gate between the back office and the galleries.