Welcome dumbfounded74. I'm glad that you found us, but I'm certainly not happy about your WW(Wayward Wife, one who cheats) giving you the reason to come find us.
Listen. Just breathe. We're here to help you. There's a lot that I would normally say, but you're in a time crunch so I want to give you the basics first.
First off, read the Tactical Primer, which is pinned to the top of this forum. That's going to give you the quickest idea of what you need to be thinking right now. Put that flash drive in a safe place, and by that I mean a place that your WW cannot find it. If you have another drive, then make a copy of it, and hide both of them in two different places. This is your evidence gold mine so treat it as such.
Normally, the advice is to never give up your sources, however since you HAVE all of the evidence, I would lead with that. Just sit her down, ask her if there's something that she needs to tell you about your youngest child. I'm certain the verbal tap dancing will start then. Ask her if there is something that she needs to tell you about the OM (Other Man). If/when she starts to lie to you again, drop the hammer. Tell her that you found the flash drive, you read it, and it and copies of it are in a safe place. And then just sit back and see where she goes.
1. Do not give up this website to her. This is your safe place. Make sure that she can't find your here.
2. Accept no blame whatsoever for her affair. No matter what she says about the marriage, true or not, infidelity is not the answer. Discussing the problems, marriage counseling, separation, or even divorce are adult choices. Infidelity is not. The affair is 100% on her.
3. If she starts to try to sidetrack you, tell her that you can talk about X later, but right now you're discussing her infidelity.
4. Make damned sure that you do not threaten her. You do not want the police called out there and her telling them that you have threatened her, frightened her, hit her, etc. Domestic Violence allegations that are lies told by waywards are not uncommon.
And last, you do not have to make one decision right now, about if you're staying or going. You can wait until you are ready to make that decision. And if anyone leaves the house, it should be her.
When you have the time, also read The Healing Library. You can find that link in the yellow box at the top left of your screen. Loads of good information there, but make sure you read the Tactical Primer before your WW gets home.
Get to reading, and let us know how it went later on. We're all here for you.