Can you paint over formica desktops and if so, please tell me how?

Yea you can paint formica is you prepare it properly.

You will need to sand it with fine sandpaper, then wipe down with paint thinner, and follow this with an ammonia and water wash. The last two steps are the most important as you need to make sure the surface is clean and free of wax.

Then you will need to prime it with a bonding type primer such as Inslx Stix which can be found at a Ben Moore store. The topcoat should be an oil based enamel if possible or if not available then use the water borne satin impervo from Ben Moore.