Delete Photo's

Yes, what Elie said. Do not reformat the card by your computer. The reason is that some cameras have proprietary formatting and if you use your computer's formatting, your card might not be recognized by your camera. 

Check your camera's instructions and look for reformatting your camera's card.

BTW, you might also want to check out Card Readers for your computer. These are NOT expensive and have a variety of benefits including not being dependant upon your camera's battery and if you have a fast USB connection on your computer, might be much faster than downloading from your camera.

Also, IF your computer has a thin slot (more typically seen on laptops), you might have a card reader built in already.