I have gone through the instructions on the Fiddler site. I have the Fiddler root cert trusted in Windows (using Windows 7), but when I connect a browser to HTTPS sites I get this:
"Fiddler's HTTPS Decryption feature is enabled, but this specific tunnel was configured not to be decrypted. Settings can be found inside Tools > Options > HTTPS."
When I check that option page, everything looks ok.
