I bought a SanDisk 4GB SDHC card for my Canon

I bought a SanDisk 4GB SDHC card for my Canon PowerShot SD870IS digital camera. It worked fine for about 3 weeks, then all of a sudden, it said "Memory Card Error".
Although I can download the photos from the camera onto my computer (thank goodness!), I cannot delete anything from the memory card. I also tried formatting the card, either using the camera, or my computer, but neither method worked.
Is there any other way to delete the photos or format the card?