for this trick to work you will need winrar
i will now assume that you have got it.
steps:

• install winrar on your machine
• now copy whatever you want to hide
• make a new folder and paste the stuff in it.
• now right click the folder and select add to archive "new folder.rar"
• now create another folder and paste the rar file in it.
• copy a jpeg image and paste in same folder .
• rename it 1 (for simplicity) and rar file as 2
• now goto command prompt :
• start>run>cmd
• enter the folder using cd command
• now at command prompt type
• copy /b 1.jpg + 2.rar 3.jpg
• wait for few seconds so that 3.jpg gets created
• type exit
• now go back to the folder and you will see a jpg image called 3.jpg
• double click to open and you will the original image
• now open same with winrar by right clicking and you will see all your hidden files

i have tried to make this as simple as possible

this is all you need to know about copy command

copy:
Copies one or more files to another location.

COPY [/D] [/V] [/N] [/Y | /-Y] [/Z] [/A | /B ] source [/A | /B]
[+ source [/A | /B] [+ ...]] [destination [/A | /B]]

source Specifies the file or files to be copied.
/A Indicates an ASCII text file.
/B Indicates a binary file.
/D Allow the destination file to be created decrypted
destination Specifies the directory and/or filename for the new file(s).
/V Verifies that new files are written correctly.
/N Uses short filename, if available, when copying a file with a
non-8dot3 name.
/Y Suppresses prompting to confirm you want to overwrite an
existing destination file.
/-Y Causes prompting to confirm you want to overwrite an
existing destination file.
/Z Copies networked files in restartable mode.

The switch /Y may be preset in the COPYCMD environment variable.
This may be overridden with /-Y on the command line. Default is
to prompt on overwrites unless COPY command is being executed from
within a batch script.

To append files, specify a single file for destination, but multiple files
for source (using wildcards or file1+file2+file3 format).

if you want to know about writing batch files just leave a comment