Below are the possible causes of this error message:

• You don't have Administrator's Privileges
  Make sure you're logging in as administrator. To install/launch VOCALOID2 you must have Administrator rights.
• DSE2.dll file is not properly installed or broken
  When there is no DSE2.dll file within the same folder where VOCALOID2.exe is (usually, C:\Program Files\VOCALOID2), or when you get this error even though there is DSE2.dll file in the same folder, please try re-installing. It is not necessary to uninstall before you re-install. Just install again over it.
• Installed older version of VOCALOID2 over newer version
  For instance, if you install MEGURINE LUKA first and then install HATSUNE MIKU next, the older version of VOCALOID2 software included in HATSUNE MIKU over writes a newer version in MEGURINE LUKA. If this is the case, 、please install the newer version of VOCALOID2 software again, by selecting "VOCLAOID2 Editor" and "VOCALOID2 VSTi" at CUSTOM INSTALL in the wizard.
• OS after Windows Vista: Compatibility Mode is ON
  Right-click on a shortcut of VOCALOID2 Editor you can find on your desktop or in Windows START menu, and select [Properties]. In the Compatibility Tab, if "Run this program in compatibility mode" is ticked, please remove the tick and click OK to apply the change, then check if VOCALOID2 Editor launches.