For Windows Vista and Windows 7 users:
Please right click on MapleStory.exe and click "Run as Administrator"
Please try out and see if this will fix your problem.
At the same time, for players having this problem, please vote in the poll which operation systems are you using.
Update: After several investigation, we discovered Avast Antivirus users still encountering HackShield: Initialization error (0×00000204) as there are conflicts between Avast and Hackshield. We advise users to uninstall Avast and use other anti virus program to solve the problem.
We would like to request Maplers to update your avast antivirus to the latest version, restart your computer system before launching MapleStory client to update hackshield.
Thank you very much for your continued support towards MapleStory.