If that is not a Norton message do not click on any buttons on that notice - seek professional help. Especially any button that says 'Fix it' or similar. There are nasties going around, some will encrypt your hard disk and then try ransom you for the unencryption key. Sometimes these come up as 'antivirus messages', sometimes as 'your PC is overheating', 'your PC is running slowly', etc, etc. If you are unsure what is causing this then get some help, else you may never see anything on your hard disk again.