This post: http://blogs.telerik.com/fiddler/posts/13-02-28/help!-running-fiddler-fixes-my-app-
explains the most common scenarios in which Fiddler "magically" fixes a site just by running Fiddler.
It sounds like you might be saying that Fiddler isn't even *running*-- it's just installed? If that alone helps, it's either a coincidence, or someone had unchecked the default setting of "Use HTTP/1.1 through proxy servers" setting inside IE's Tools > Internet Options > Advanced. (That's the only change to the browser that Fiddler makes upon install.)
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.