It is fairly well known that on Windows, when you delete a file (or folder which may contain any number of files and subfolders possibly containing other files and subfolders and so on), Windows just deletes the filesystem pointer to the node corresponding to the file or folder in the corresponding filesystem tree structure.
So, I would like to know, if someone uses 256-bit AES as available as part of the implementation 7-zip, TrueCrypt, or BitLocker, or any other conforming software, and uses, say, a 20-character passord which uses uppercase, lowercase, as well as numeric digits, then the encrypted folder should would be encrypted with enough strenght that nobody could open it unless they had the password.
But my concern is that in creating the encrypted folder, the old data would remain unencrypted on disk, and could therefore be recovered with a program used to undelete files (and I'm sure there are several of these). So, which of the encrypting solutions I've mentioned actually zeroes-out the old data?
Even if so, I don't even know if this is a real solution. If the user has only moved the file across the same hard-drive throug cut-and-paste operations, then theoretically Windows would not have copied the information contained in the folder, just updated the pointer to it. But if the user has copied the information from one drive/partition to another, than the information contained in the folder would also end up being copied, and the information could still be recovered via a Windows undelete program.
Furthermore, I don't know whether there are any circumstances where the user doesn't do anything but Windows copies the data associated with the folder, leaving an old copy of the data behind (perhaps defrag, I don't know of any other situations).
So, my question is, how secure are 7-Zip, TrueCrypt, BitLocker, or any other preferable viable alternative, in fully deleting the original folder after it has been encrypted. If none do, then are there any programs available which will zero-out any unused portions of a drive/partition without reformatting it?