If you just need to casually hide one or more folders, use Free
Hide Folder. It's a menu-driven and password protected tool that
creates obscurely-named, hidden folders to hold your private data.
But it's not secure--Directory
Opus, an ordinary file manager, displayed the hidden folder.
And if you search for a filename in the folder, or part of the
filename, say, *.JPG, you can find the files and their locations.
If you want the folders hidden and secured, use TrueCrypt.
It's a free, powerful tool that creates an encrypted volume you use
like any other drive--except that only you can access the files. To
avoid losing your data, it's essential you read the tutorial.