[kevina]

I assume you really intend this:

((Due Date <= 2 weeks from today AND Due Date > today) OR (Begin Date <= 2 weeks from today AND Begin Date > today)) AND Flag != Completed

This is slightly different than what you specified (the extra parentheses make the Flag criteria always required and only on of the other expressions). If so, below is how you would define the Advanced Search:

Flag != Completed
    AND DueDate <= #2 weeks from today#
    AND DueDate > #today#
        OR BeginDate <= #2 weeks from today#
        AND Begin Date > #today#