A five-time loser at the Oscars previously, Winslet triumphed for her role as a former concentration camp guard in an affair with a teenager.
Heath Ledger is the supporting-actor winner for his demented reinvention of Batman villain the Joker in "The Dark Knight," becoming only the second actor ever to win an Oscar posthumously.
The supporting-actress trophy has gone to Penelope Cruz as a tempestuous artist in Woody Allen's Spanish romance "Vicky Cristina Barcelona."
The evening is following the script of earlier Hollywood honors, with "Slumdog Millionaire" steamrolling through. Its awards include adapted screenplay, cinematography and both music awards, for score and song.
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