AES Protect is a portable tool for encrypting single files using a passphrase and AES.
The program arrives as a sub-50KB download, yet still somehow manages to squeeze in three programs. AES Protect is for general use; AES Metal gives expert level control, and enables managing features like initialization vector and key; AES Command is a console-based tool which combines AES Protect and AES Metal functionality.
The interface is simple. Select the file to encrypt, the destination folder and file, and the passphrase (twice, for confirmation). Click Start and the file is encrypted within a few seconds.
Choose the same file when you need to decrypt it, and enter the passphrase again. Click the Encrypt button to toggle to a Decrypt operation, and click Start to retrieve it.
Beware: there's no back door here, no way to recover a file if there are problems. Test the program carefully to make sure you understand how it works, and don't encrypt anything unless you've a backup somewhere.