Select CHEATS in the options menu and type in (confirm with [Enter]):

 MR HAPPY            No dressage failures
 BIGHEAD             Big head mode
 PHAR LAP            Unlimited energy round the cross country
 CHRISTMAS SNOW      ?
 GENIE IN A BOTTLE   Unlock all tracks and horses
 NEW YEARS EVE       Fireworks
 TANTALUS BIG AIR    Ballistic rider