How do I get the big DEMO off the screen?

Factory reset it,

Power off completely
2. then hold down the volume down button
3. holding the button press the power on button.
4. if done properly you should have several android boot options
5. select "Factory data reset"
6. Done

Note: if the volume down did not work, try the Volume Up button

Go to the system settings and look through them to see if there is an option for demo mode. Also, go to Settings>Apps>All, and see if there is an app called "Demo." If there is, tap it and tap Disable, if possible.

Return it to where you bought it, someone has customised the start up screen.

Turn it off demo mode in the settings. Your user manual will have details on how to do this

below is a link if you read #14 this will give you instructions on how to install the software that has removed it i have not registered so i can not confirm the software works.you will need to down load a file called SystemPack.APK and run it from your tablet as ypu would a app

If there is an options menu go and find the demo there should be one. I think that would be the place. Hope this helped you.

in the middle of the screen

Which device?

You might find the answer here:
ARCHOS 101 User Guide