Transfer photos from iPhone to PC.

When I transfer photos from my iPhone to my PC, I connect the USB cable to my PC and the iPhone asks “do you trust this computer?”. I mistakenly replied “no” and now I can neither look at nor transfer any pictures contained in the iPhone. Worse, on subsequent connections, the iPhone no longer asks if I trust this computer - it seems to recognize my PC as untrustworthy. How do I reverse this mistake so I can once again backup my photos to my PC?