My boyfriend has been messaging another girl

And he's lost yours.

So fess up. But be prepared for a fight.

There's no such thing as an accident (he left his Skype open)

He wanted to be caught. This is his way of getting rid of you.

If I leave my mail open in an envelope does not mean others can read the letter.
If I leave my dairy around - does not mean others should read it.
Behave the same way with e-mails, skype and sms.
Correct behavior does not change just because the way of communicating change.

I have to agree with Punkey though, Just because it wasn't right to read the messages doesn't mean he didn't know she would.

He wanted to get caught.

This doesn't make sense to me. I can't imagine ever doing this on purpose. If I want to end a relationship, there are easier ways to do it.

Some people are just passive aggressive. My ex husband cheated and left his phone around, and made it very obvious he was doing something he shouldn't on text. So I looked, and found what I expected. I would have very much preferred him to be honest and just say he wanted out. Some people are just cowards when it comes to being honest.

I've fallen asleep in front of a computer a few times over my life.

Granted, I had nothing compromising on my screen (sometimes the screen had a half-composed a2k message), but the falling asleep was entirely unintentional and probably would have happened no matter what was on my screen at that moment.