I guess it's a little too late for prevention but... you put your presentation with your Poke collection :surprised
My suggestion to you is to just laugh it off because there is no sense denying it at this point. A "how did THAT get there" probably won't work... but, I don't know how long USB keys/cards have been out; you could just say that you have a lot of really old stuff on there and you don't like it as much as you used to (even if it's not true). If people still have a problem with it, just say "who cares". It's not like you are less of a person because of it. You'll truely know who your friends are at this point I'd say. But the key is not to act embarrassed even if you are embarrased as hell... take it lightly and... laugh!
EVERYONE has their own guilty pleasures and if someone claims that they don't, they are probably lying :P. Ya just hope that stuff doesn't leak out. Just keep thinking that... it could have been worse. It could have been some other collection...
If you're thinking that it's easy for me to say, take this. I'm 19 now, but when I was in grade 9, I wore a Pokemon shirt to school once. And not just any Pokemon shirt... it was blue and orange and stuck out like you wouldn't believe! I dunno what I was on that morning that made me grab that shirt, I just usually wore it at home back then. But realizing it was too late once I got there, I knew I was in for a tough day. So whenever I'd hear "Haha, you like Pokemon" I would sarcastically reply "YA! That's right! POKEMON YEA!" and then laugh like an idiot like I was making a big joke. Normally I would have just froze and went as red as a tomato. I could have easily had my jacket on all day or switched to my gym shirt for the rest of the day, but that would have shown that I was hiding something and showing embarassment, so I toughed it out the whole day. Needless to say I never wore that shirt in public again. Things died down after about a week and things were back to normal again. That was Junior High for me, maybe High School would be a lot less forgiving. Yup, I'm surprised with how much crap I put up with through grade school, kids can be so cruel! But we all have faced it and we all survived it!
So whenever I have to do a presentation in college with my laptop, I go through everything making sure there can be no slip ups! Wallpaper changes, zipping, encryption, hidden files, I lock everything down, do what I have to do, and return everything to normal again.
Anyways, good luck to ya man! Tell us how everything goes!