As Normis say’s at this post:
http://forum.mikrotik.com/t/rb2011uas-2hnd-in-netinstall-without-serial-cable/76620/4
Make sure your Windows PC has Firewall and Antivirus turned off, and that you run Netinstall “As Administrator”.
And here how easy to do it 8 ways.
http://www.sevenforums.com/tutorials/11841-run-administrator.html