Unfortunately, I don't think Wireshark can see traffic that doesn't leave your PC, and since Fiddler runs locally it probably won't help to use Wireshark.
From a command prompt, you can type netstat -a -o
to verify that Fiddler is properly listening on 127.0.0.1:9999.
From a browser, you might try visiting http://yourPCName:9999/
and verify that it doesn't load anything.
You might look in Add/Remove Programs for any recently installed software that might be interfering with your network stack.
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