What exactly is happening when you plug in your powershot and turn it on? Are you trying to use third party software?
I have a canon powershot that works fine with windows 7. I don't use any software besides windows to recognize the camera. I just open it up as a folder and then navigate to my pictures for copy/pasting.
Plug in your camera and turn it on. Go to your device manager and look under "portable devices". Does your camera show up there?