You will need some recovery software.
It works in conjunction with the card reader on your computer.

There are actually hundreds of such programs.
Some are free, some shareware and some commercial programs.

Here is a link to a Google search.

I have not used any of these myself so I can not recommend any particular one.

Just go down the list until you find one that looks like it will do what you want:

http://www.google.com/search?hl=en&q=memory+card+recovery&btnG=Google+Search

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