Beware, though: most usb viruses/trojan/whatever hide there.
If you also see an autorun.inf automagically appearing in the card, it's probably because it's been in an infected computer.
If I were you I'd disable autorun on windows (and google for it, since microsoft solution doesn't completely disable it and leaves room for the virus to propagate).
Edit: I'd also configure explorer to show hidden files and not to hide the file extensions. You know, windows defaults are there to allow malign code to spread, so it's a safe bet to reverse those defaults.