Till, I suggest rethinking the order of the main menu. Make the order by the most used buttons. When login have the selected button Sites and not emails. People manage sites more often than emails. Make buttons in this order: Sites, Clients, Emails, DNS, Monitor, System, Tools, Help Notes: Change Client to Clients, Email to Emails. System should not be first button because once the server is configured, it's not used as often as Sites.
You probably already know how to do it but... You can change the order of the menu items in de ISPConfig database. In phpmyadmin choose "dbispconfig" as database and select the "sys_user" table. Click on "Browse". Edit the row of the user you want to change and modify the value of the "modules" field. Grtz, ArnOS.