Agera One :
http://www.wikihow.com/Connect-a-Desktop-to-A-Laptop
http://www.youtube.com/watch?v=89dof7m2xZM
These 2 links do no solve the issue. First link is to share data, and the second link is opposite to what was asked.
I don't think it is possible to use laptop monitor as your desktop monitor. The only way I know is to use your laptop to do a remote access on you desktop (assuming both are running windows). Easier just buy a monitor.