My advice would be to download the Service Pack (Presuming it's SP1.1 your installing) to your desktop, right click on the file, go to properties and where it says security, unblock the file.
Might be worth at this stage going to Start and typing in %TEMP% and deleting the files in this location...
Then make sure AutoCAD 2013 is closed, right click on the file and Run As Administrator... If you get any pop ups that say Retry, Cancel or Ignore, use Ignore and it will bypass the issue.
Check on your control panel that SP1.1 has installed by going to Start > Control Panel > Programs > View Installed Updates and hopefully for AutoCAD 2013 it says AutoCAD 2013 - English SP1.1
You may find that if you still cannot do this then do a complete clean uninstall of AutoCAD 2013 (http://usa.autodesk.com/adsk/servlet/ps/dl/item?si
Hope that helps?