Can I hide folders???

I have an external Hdd connected to my mac, and the Hdd contains a folder that came on it to make sure it runs on a mac and a windows (I need it to work for both) but sisters on my computer dont know any better and keep opening these important files and I'm worried they'll mess something up.
Is there any way I can "Hide" files...or maybe even Lock them?

-Emoguy

Most people here will suggest you create an encrypted disk image withg a pass word. I have never done it, but I believe you use the disk utility application. Choose new image, then set the size, the encryption and the format.