I had two W's; one (calculus class) I took later and made an A, (dropped it b/c I had just come back to the U.S. from spending 18 years overseas; didn't really "get" the U.S. math/symbols system; was different than the commonwealth system; I needed to back up and take a lower level class first.) and the other one was a law class I just dropped because I changed my major from public administration to something more compatible with my med school goals. I was asked a couple of times about the two Ws. I just explained, without making excuses. Wasn't a problem. If you let them know that you work in research a lot, and that you obviously performed well later, it might not be too big of an issue. If they don't seem comfortable with it, you can mention what you "learned" from the experience. (i.e...when I took constitutional law, it required SO much of my study time, that I couldn't focus on biochem, so I dropped the law class). You can say that you learned not to overload yourself, or that you realize that you can accomplish more than you'd thought and maybe didn't need to drop so much, or that you were happy you came through school well, even with a heavy extra curricular load, because the research was important to you.... If you've made good grades in the same classes, they'll see you can do the work, you just don't want to come across as gunshy about difficult classes. Just be confident and gracious and don't panic if they ask, and come up with a polite answer that shows you were happy with the decisions you made. Good luck!
. And one of those 2 accepted me 
