Chalala, there are two ways I know of for someone to remotely view your entire screen.
The first, Remote Assistance, is built into Windows XP. You can disable it via System Properties under the Remote tab.
The second is through
Virtual Network Computing (VNC). The way I've seen it used, some VNC software is installed on the computer to be monitored (your computer) and then accessed from a remote computer via some permissions set locally. I'm fairly certain that
some software must be installed and running on your machine in order for someone to be able to see the whole screen.
You should look into getting rid of remote access to your computer right away, because both of these methods also allow complete control of your machine (think viruses, bank accounts, etc.) Good luck, and let us know what you find.