I don't think Peyton overreacted AT ALL; I think she had every right to act the way she did. Brooke was absolutely dreadful this season, and she ended up coming across as one big hypocrite and jerk - with things out the Breyton fighting even.
Had Breyton not been fighting or had Brooke not been such an uncalled for, over-the-top bitch, I don't think Peyton would have reacted that way. It was about so much more than just Nathan and Brooke hooking up. Brooke went out of her way to treat Peyton like she was this lying slut who intentionally backstabbed her when Brooke should cast herself on the same level.
I don't care that Pathan were broken up - as Peyton said, they broke up all the time. Brooke hooked up with her bff's boyfriend like two seconds after they broke up. Girl code is still in effect. In my opinion, what she did was no better/no worse than what Peyton did.
And I am so tired of people using Brooke being drunk as an excuse. That'd be like if someone was drunk and murdered someone - well, that person shouldn't be held accountable b/c they were drunk. Yeah, I don't think so. Like someone else said, alcohol loosens you up; it doesn't make you unaware of your actions. I have been drunk many times and I may have said/done some inappropriate things, but I was aware of my actions every time. Brooke pulls the "I was drunk" line way too much. And the way she tried to cry victim after and then blaming Peyton for Chase breaking up with her. Please - STFU, Brooke! Brooke seriously needed to grow up, own up, and take responsibility for her own actions and quit blaming everyone else.
Let's also not forget the horrible things Brooke said about Peyton's mom. That was some of the lowest, most disgusting things Brooke has ever done. You could hear the hurt in Peyton's voice and see the betrayal on her face when Peyton brought that up during their fight at her house. I think that's where the root of most of Peyton's anger/reaction came from. I was wanting Peyton to punch Brooke again - she deserved it the first time, but I don't think once was fair enough.
As for Haley's reaction - I just found it more of a comic relief b/c of her hormones. Besides, along with the emotional craziness due to pregnancy, who wouldn't go a little "crazy" after finding out your husband slept with a good friend of yours, not to mention MOH - even if it happened before you knew each other/got together. I admit I would; my man better share his roster with me. It's not like Haley really went off the handle and went off on anyone or even held onto it in a cruel, hateful way. It was supposed to be funny and not taken so seriously. For me, that's exactly how it came across.