This happened to me in about 2007, and it turned out that for reasons known only to the universe, it was caused by using an older version of Internet Explorer, and was cured by updating my version of Explorer. Obviously if you are using Windows 7, it isn't the same version of Explorer that was the culprit in 2007 so this is a long shot, but you might try making sure that you are using the latest version of Explorer and other utilities that are tied with Windows 7.