A 1. This is because the latest versions of PLATON.EXE require a current version of the library SALFLIBC.DLL. If you are running PLATON from the PWT taskbar, you will need to download the latest version of PWT. Place your new version of PLATON.exe in the same folder as PWT.EXE and it should run OK.
If you are running PLATON from WinGX or any other way, you will need to download the latest version of SALFLIBC.DLL from here . The file should be placed in the same folder as PLATON.EXE. If using WinGX to launch PLATON, then it should be in the WinGX binaries directory (by default c:\wingx\bin).
A 2. This is because there is a previous instance of PLATON running (which may not be visible). This happens because you have not stopped PLATON properly, but just closed the Graphics or Text Windows (this DOESNT stop the program, unlike with other Windows applications). Use the Task Manager for Windows (right click on task bar) to kill all previous instances of PLATON and all should be OK.
Recent versions of the Taskbar (1.02.2 or later) should overcome this problem because they explicitly terminate all previous instances of PLATON before starting a new job.
A 3. This is because PLATON cannot find the file CHECK.DEF. The environment variable CHECKDEF is either not set, or is incorrectly set. The value should be a character string comprising the fullpath name of the file "check.def". See this link for further information about setting environment variables.
A 4. PWT cannot find the PLATON executable. Put a shortcut to PWT on your desktop and right-click this icon to get the Properties Dialog Box. Set the "Start in" directory to c:\pwt (or wherever you have installed PWT/PLATON).