Unfortunately, I don't know of an app that will let you browse your pc, as well as, move files back and forth. That is one thing I miss about my BlackBerry, was that it's sd card would show up on my computer and I move files to it that my BlackBerry would be able to read and email.
There are some wifi apps that let you move things back and forth between your pc and your iPhone, but they obviously aren't connected via cable. Air Sharing, Briefcase, etc.
But after looking into this just now, I found one called FlashDrive by Readdle that supposedly lets you move all compatible files to your iPhone from your pc. It also gives you the ability to email these files through their sever from your iPhone. I'm going to give this app a try, looks interesting. I'm not sure what kind of security or encryption Readdle uses to keep your documents private, but I'm sure you could contact them for specifics. This looks like an app that could be really useful.