Maddie McCann's Parents Keep Hope Alive

The parents of Madeleine McCann -- the 4-year-old British child who went missing during a vacation in Portugal two years ago -- are keeping hope alive that they'll see their daughter again.

Gerry and Kate McCann were interviewed for the first time in the United States on "The Oprah Winfrey Show," talking about their continued search for Maddie. The interview airs Monday.

"Do you let yourself go to the worst sometimes?" Oprah asks in the interview.

"I mean, I do. I think it's natural," Kate admitted. "I know people mean well and they say, 'Don't let yourself go there. It's not gonna help going there,' but as a mom, inevitably, there are times I do."

But the couple is optimistic that a new image of Madeline -- enhanced to show how she would look at 6 years old -- will spark new leads.

In the interview, Oprah also asks the McCanns if they believe someone was watching their family, how it felt to be vilified by the world after their daughter's disappearance, and if they will one day give up their search.

Madeleine was left alone with her twin 2-year-old brothers in a vacation apartment while her parents had dinner less than 100 yards away in a nearby restaurant, reports CBS News correspondent Elizabeth Palmer. Both parents, who are medical doctors, were initially suspected by Portuguese authorities, but were never charged.

Two years later, says Palmer, the trail has gone cold in Portugal. The vast police investigation turned up few clues, even in the room where Madeleine had been sleeping.

One theory, from criminologist Mark Williams-Thomas, who reviewed the Portugese police files, is that Madeleine went outside.

"I believe on that evening, she woke up, heard the twins crying, went to look for Gerry and Kate -- they weren't in the apartment," Williams said. "She walked out of the apartment through the insecure patio doors -- which she was capable of opening, and that's where she looked for her parents."

The search went on for 14 months, but was called off last July. No evidence of a crime has been found, and no suspects, including a local man, Robert Murat, were ever charged.

"Within three hours," says Mark Williams-Thomas, "74 percent of children abducted by strangers are dead, and within the first 24 hours, it's 91 percent. So, statistically, it tells you that the chances are she's dead, and I believe she probably is."

But the McCann's are keeping hope alive. Kate McCann keeps her daughter's room ready for her return.

Oprah asked Kate McCann if she still talks to Maddie.

"I tend to open and close the curtains morning and evening and just say, 'Hello,' really," Kate told Winfrey, "just tell her we're still going and, you know, we're going to do everything we can to find her."

Last July, Portuguese authorities halted their 14-month probe into Maddy's disappearance after detectives said they'd uncovered no evidence of a crime. The case was to stay on hold unless new evidence emerged, authorities said.

If you have any information about the whereabouts of Madeline McCann, please visit the search Web site, Find Madeleine.