You can get a nice prog called DScrypt. If you search for "46 best freeware" utilities in google, there's a page with them listed and it is on there.
I don't know if there's a size limit, but I use it to secure files and folders. Very small file, you can keep it on USB drives and things. You open it up and set it to "encrypt" mode (which is does by default) and drag files. You give it a password, confirm it, and then it's secure. You open up the program again and change it to "decrypt" mode, drag the file, put in the password, and it's back to normal.
Again, I dunno if it can handle multiple gigs worth, but it handles my small text files with ease. I assume it can do the same with larger stuff but I dunno.
Give it shot.