I just don't understand this 'critical window' thing. Yes, pups should be socialized--but if they're not, and the window is "closed," they could never be rehabilitated. If that were the case, how on earth are all these rescue dogs doing so well?
Anyway, I'm on the opposite end from most of the posters. I got Aspen when he was 10 weeks old. He was very friendly and we took him with us wherever we went. He met adults, kids, other dogs, cats, went to puppy kindergarten, etc. Then between 4-6 months (after the so-called "critical window") he started to change. Nothing 'bad' had happened to him, he just started disliking people he didn't know. He'd go nuts. At the age of 2 he stopped liking other dogs he didn't know. We now know he has some neurological issues (and is on meds for them), so maybe this contributed to his behavior. I don't know. He's now 4 1/2 and really a wonderful dog. He goes to agility classes and we even take him to demos at the County Fair, Humane Society events, or wherever else our trainer wants an adorable dog to "show off." He's very good at these venues and is very quiet. His preferred method of dealing with scary things in the world now is to back away rather than stand his ground. Distraction (in the form of a treat, squeak of his ball, or softly calling his name) works wonders if he's getting antsy. He still doesn't like strangers and takes awhile to warm up to people and other dogs, but if given enough time he does eventually. Exactly like others stated above, we tell people to ignore him. Yes, at home he'll approach you barking and carrying on and then dash off--but don't look at him, don't talk to him (and for pete's sake don't try to sooth him by telling him it's all right!), and don't try to pet him. This has never failed.
Don't give up! Your dog may never be a "butterfly," Aspen certainly won't, but they can learn to deal with things. Very well, in fact!