That's what I'd say!!!
As you have done a good job in troubleshooting it, my guess is there is a problem with the card, or possibly formatting the card. In the Canon onboard menu, there is a choice to reset or reformatt the SD card.
Of course USB off all you keepable photos onto your Win-7 first, then with the camera see if you can reformat the card. Might also see if when connected by USB, if you can save a photo from your computer to the card.
Other than that, it's not worth keeping a flaky menory unit.