In my old University, women were common in the first year of Comp Sci, as it was the easiest department to gain entry to for them as females because of quotas or whatever positive discrimination is called.
However in the second year, they would typically transfer out to another department they were more keen on, but probably wouldn't have won a place at initially.
"By 2010, one in four new jobs will be “technically-oriented,” or involving computers. However, women still lag far behind in earning computer technology degrees and working in computer technology-related professions. High school girls represent only 17 percent of computer science AP test takers and college-educated women earn only 27 percent of bachelor’s degrees in computer science (down from 37 percent in 1984) and 16.3 percent of doctorate degrees in computer science. Overall, women comprise roughly 20 percent of IT professionals. (“Tech Savvy: Educating Girls in the New Computer Age,” American Association for University Women, 2000)" [1]
Barring the "down from 37 percent" part, it tells me that 17% of female high schoolers take AP computer science, but actually a higher percentage of women end up with a CS diploma. It's very thin connection but that does raise a question that affirmative action lets in more women.
[1] http://www.civilrights.org/equal-opportunity/fact-sheets/fac...
Stanford doesn't separate undergrad admissions by department. When I applied, the application didn't even ask what my intended major was. All Stanford undergrads start out as "undeclared", and many don't choose a major until sophomore year.