Your web browser's User-Agent string contained data which did not match your actual web browser. This is typically associated with malicious activity on your computer.
To resolve this problem, first clean your computer of viruses and other malware. Then reset your browser's User-Agent string to its default value, or use a different web browser.
Internet Explorer: Update to IE version 9 or higher. If you are unable to update, remove invalid entries from your registry.
Firefox: Reset your User-Agent string.
If you are using a work computer and are unable to make these changes yourself, contact your network administrator for assistance.