Honda recalled the ignition switches on certain 1997-2000 vehicles. Your car may have one of the defective switches. Call your Honda dealer with your VIN and ask if your car was recalled for this problem. If so, you or the previous owner may have missed the recall and not gotten a new switch. And even if a defective switch was replaced way back then, the replacement ignition switch could still be causing your problem due to too many keys hanging from your key ring.
More info here:
http://www.hondacivicforum.com/forum/mechanical-problems-technical-chat-8/ignition-switch-recall-problem-88665/