I think there may be a problem with the start page which is rendered using the system built-in version of Internet Explorer. Could you try updating the system's Internet Explorer to the latest version? It's 8 on XP and 9 on Vista and up. Also it would be good to make sure the system is fully patched with all patches available on Windows Update.
If that still doesn't work you could try disabling the start page completely. Try setting the registry key 'HideStartPage' to the DWORD 1 in:
HKEY_CURRENT_USER\Software\Scirra\Construct2\Preferences
Does that help?