The most likely explanation is that this isn't related to Fiddler at all, and instead something that you've downloaded installed malware like Browser Safeguard or GeniusBox and that is interfering with your browsing traffic.
You may be able to find the malware using the Add/Remove Programs Control Panel. Alternatively, check your IE proxy settings to find out what port your proxy is set to, then look to see what process is running on that port (e.g. netstat -o) and then find the executable in question and kill it.
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