Queen Elizabeth II has given a Canadian businessman the go-ahead to drill for what could be one of Europe's largest onshore oil deposits -- right on the grounds of her cherished Windsor Castle. The would-be driller, Canuk Exploration Ltd. director Desmond Oswald, promised yesterday that the exploratory drilling will be a half-mile distant from the queen's ancestral home and won't mar her view of the family parkland. He didn't precisely address what would happen if the hoped-for 100 million barrels of oil starts gushing out. "I don't believe you can get 100 million barrels of oil just through a small pipeline," Windsor Mayor Dennis Outwin fretted. "It's bound to lead to Windsor Great Park being turned almost into a second Dallas."