Some well known SEO/backlinking tools set a proxy in windows rather than applying it to itself. This can become troublesome when you come to use any other aplication that relies on the windows proxy settings.
If you have been getting login problems the most likely scenario is that you have a windows proxy set that is no longer working, to remove it simply follow the steps below:
1) Go to your control panel
2) Open Internet Options
3) Go to the Connections tab
4) Click the LAN Settings button
5) Uncheck the option labelled "Use a proxy server for your LAN"
6) Click OK
Alternate way:
1) Load up Internet Explorer (this MUST BE INTERNET EXPLORER and not ff or opera or any other browser)
2) Go to Tools => Internet Options
3) Go to the Connections tab
4) Click the LAN Settings button
5) Uncheck the option labelled "Use a proxy server for your LAN"
6) Click OK
And that is it, you should now be able to login without an issue.