While working with a client system we found that the Internet Explorer or any other browser application kept having issues with accessing the Internet. Reseting the browser settings would temporarily resolve the problem but it would quickly return the moment we would leave the client.
It turns out the client was using Juno as their e-mail application. As the user would review their mail they would occasionally get a message that had a hyperlink in the message to a web site. When clicking the link Juno would automatically open by change the proxy settings to force the browsing through their system.