This message occurs with Microsoft Vista users. Vista is currently not a supported operating system for Asheron's Call.
It has been reported that updating to 'Microsoft Windows Vista Service Pack 2' can remedy this error.
When launching AC on Vista and Windows 7 be sure to right-click the AC icon and select "Run as Administrator" and choose "Yes" when prompted to allow the program to make changes in the following dialog (be sure to supply admin credentials if required).
While Vista or Windows 7 is not yet supported with AC, to assist customers who are using Vista, there is another additional workaround. Please download the following .ZIP file and extract it in to your AC folder, then try launching the game. We cannot guarantee this will solve the problem for everyone, but this should help in many cases.
Please keep an eye on the Official Asheron's Call forums for updates, as testing for Vista and Windows 7 compatibility continues.