just take my advice... it's not THAT big a deal. It may seem so at first and you can make jokes about this, but if you really think about it... what is the big deal. And this is coming from me out of all people, apply85 and some others can confirm this.
What I would do? First I wouldn't have checked her open windows's. I would've told her that she can finnish her things in her fb etc, that you only saw windows open and didn't want to check them out. Secondly, I would've treated her at present what she is now, not what she might've been to other people or whatever.
I would grab her ass like I would any normal day (if I was in your shoes) and eat her, fuck her and whatever.
Don't take things for granted but be open minded, I think you wanted this girl all for yourself after just couple of weeks seeing her, these things happend and that's why adults usually don't take things for granted and just see how things build up.
I think you just generated a semi hell of your own in your head and my advice is just to forget about it.
I think that if there comes a time when you two both think you are for eachother she will come forward and tell you about her history... then you can be honest to her and tell her that "hey, I checked your computer..." and make an end to it there... and something after that
Just take a deep breath.. don't kill her yet