EXECUTION COPY $437,500,000 364-DAY CREDIT AGREEMENT AMONG AON CORPORATION, AS BORROWER, THE LENDERS, BANK ONE, NA, AS AGENT, AND CITIBANK, N.A., ABN AMRO BANK N.V. AND FIRST UNION NATIONAL BANK, AS SYNDICATION AGENTS DATED AS OF FEBRUARY 8, 2002 BANC...