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I had a similar problem with a bunch of cards. Apparently, the computer was corrupting things. I was able to use PC Inspector to recover the jpegs on another computer. I also used a disk wiping program to wipe all my cards just in case.
After the card had been in the bad computer, the camera said it needed formatting. I didn't format, but used PC Inspector in the good computer to rescue the files.
I tried everything I could figure out, but putting the card in the bad computer screwed it up. I thought it was the card, then the camera, but through a process of elimination I found the problem to be the computer. Now the computer is used only for occasional web browsing. The previously "bad" cards now work in the previously "bad" camera.
The symbols you are seeing are the same ones I saw on my cards. I thought maybe a virus, but several scans using internet based sweeps found the computer was clean. You can try reinstalling the operating system and see if that cures it.