Is fever of around 40 deg centigrade considered to be too high? And what is the best remedy? Does the keep head cold and body warm theory apply? Thnks

Antipyretics. Acetaminophen and Ibuprofen are good antipyretic meds and will bring your temperature down. Make sure you find out what's causing your fever, you may have an infectious disease that needs proper treatment, i.e. Antibiotics.