I was in a similar situation as you at one point, my girlfriend had just moved in with me and she got to see how much I really used my PC. In preparation, BEFORE she moved in, I swept all of the PCs for 'stuff'. I figured we were living together, working at the same company, and going to the same school, our lives were completely intertwined at that point, and that it was best to just get rid of all of the 'stuff'.
After moving in she would give me a hard time about all of the time that I wasn't paying attention to her by casually stating "There are sometimes that I think you like your computers more than you like me" ( there were 15 in the apartment in the second bedroom). Then she started getting inquisitive... "What ARE you doing when you are using your PC?" which I would poetically reply "I don't know, stuff" (Just a warning, that's really not the best answer).
One day, I'm cleaning up the hard drive on my PC, searching for old files and deleting the ones I didn't want to keep. She came in and asked "what are you doing?" I knew the coast was clear, because I already swept any spicy files BEFORE she moved in.
This particular day I was searching by file type, thinking I would just get rid of files that I didn't need that I had downloaded. This was the early days of the internet, so if I saw a cool program or picture, I felt compelled to save it because there was still the fear that this internet thing would 'go away' at some point.
Ah, the folly of youth... I invited her to sit down and join me, figuring that this would be a great opportunity for her to see what I was doing while I was using my PC. We went through 100 or so pictures together, all of family, and friends, and put them into folders so they were easy to find later. I stumbled on a picture named something like "Christmas Time.jpg" and figured it was a picture of me and my family when I was a kid, or a corny x-mas graphic.
I think I remember saying something like "See, were having a good time in front of the old PC. Hmm, let's see what this picture is honey". I double clicked the picture, and Windows opened a full screen preview of it, a girl naked in a santa suit in a VERY compromising position. In an instant I closed and deleted the picture. I had hoped that the whole process was too quick for her to realize what happened, or she had turned her head to look at the cat momentarily. In another instant I turned and looked at her to gauge her reaction. Her mouth was agape, and the color had sunk out of her face. It was too late. She had seen it all. The damage was done. Without saying a word, she got up and left the room. She didn't even give me time to try to talk my way out of it.
I got the silent treatment for about a week after that. To make matters worse, we were living together, working at the same company, and going to the same school, our lives were completely intertwined at that point.(DOH!) It seemed as if there was no way of ever living it down. I was sure she was on the verge of dumping me and moving back in with her parents.
Fast forward ten years... The whole incident wasn't as relationship busting as I had originally thought. We've been married for several years now, and have a beautiful, super smart child together.
The moral of the story is that it doesn't matter how you do it, you just need to get rid of ALL of the spiciness on your PC. If you leave any of it, there is a chance that you will get complacent and forget about something, or too proud and think that she will never find it. I guarantee it that she will find it, and it will definitely not be in the situation of your choosing. Just imagine her saying the first time you have a holiday together, "Hey mom, check out these great pictures we've got on our PC", right before inadvertently opening the illicit folder...
Back to the folly of youth... Looking back at the situation now, I realize that when I downloaded the free files in the first place I was just a desperate nerd looking for any outlet or chance at even one sided or virtual-ized interaction with a woman. I didn't realize it right away when my girlfriend moved in, but this was my chance to be with a woman. A real woman. After all... she liked me enough to want to share a house with me, there was no telling what would come of our relationship.
So congratulations man, you've made it. Don't be an idiot and screw it all up by holding on to any fictionalized ideas of what a woman should look like or do when a camera is pointed their way. You've got a real one that is going to start climbing in bed with you every night in the near future, so make the most of it. I suggest spending several nights and possibly a weekend or two alone BEFORE she moves in, completely deleting EVERYTHING that is even remotely 'spicy'.
How to round up the files? If you are using windows, you can use the built in search function. There is an option to search for 'Media" or "Movies and Pictures" since windows XP, just make sure to change the option to expand the search to look in system and hidden folders as well.
To catalog all of the images on your PC, use something like Picasa or Shotwell to find all of the images on your machine. These programs are fast and free, and will find every image on your PC.
Best idea AFTER you figure out your stray file issue, and you want to hold on to the illusion of what a woman may be? Use something like Virtual box for 'play' time. That way when your conscience rears it's head and your future wife wants to look at what your doing on line you will only have to delete ONE file.
In closing, you will have to ask your self the following questions:
How do you want to run your relationship?
Do you want to build it on trust and respect? Or do you want to build it all on a foundation of deception?
Is a bird in the hand worth a folder full of smut?
After all, would you really keep a stack of issues of Hustler and Penthouse laying around for her to stumble on?