This issue may be caused by your proxy settings. Try the following:
If you are behind a Proxy Server you will need to make a change to your browser.
For Firefox: Select Menu Item Tools >> Options
Select Advanced Tab, then Network
Under Connection Select the Settings Button
Select Auto-detect proxy settings or use manual settings and specify your proxy sever
For IE users: Select menu Item Tools >> Options
Select the Connections Tab, LAN Settings
Select Use a proxy server for your LAN and then specify your proxy name and port (Usually 80)
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Isaac AsimovWorldWide Telescope
Microsoft Research