Yes, DESKTOP is a Scrabble word!

Letter values in Scrabble US: D 2, E 1, S 1, K 5, T 1, O 1, P 3.

Is it a valid word?

DESKTOP is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the top of a desk
(computer science) the area of the screen in graphical user interfaces against which icons and windows appear

Also: screen background.