Kerry: Bush Lets Groups Do 'Dirty Work' BOSTON - Sen. John Kerry accused President Bush on Thursday of relying on front groups to challenge his record of valor in Vietnam, asserting, "He wants them to do his dirty work."   Defending his record, the Democratic presidential candidate said, "Thirty years ago, official Navy reports documented my service in Vietnam and awarded me the Silver Star, the Bronze Star and three Purple Hearts."  "Thirty years ago, this was the plain truth...