New Vaginal Gel May Protect Women From HIV By LAURAN NEERGAARD    WASHINGTON (AP) -- A chemical specially designed to thwart how the AIDS virus invades during sex offers scientists a new lead in the long quest for a vaginal gel that women could apply to protect themselves when men don't use a condom.    The experimental drug isn't ready for human testing yet, but it provided potent protection to female monkeys exposed to large amounts of an AIDS virus, researchers reported Thursday in the journal Science...