>Suppose i use: DECLARE LONG ShellExecute IN Shell32.DLL
>
>1- Will the expression above work under
>any version of windows ?

The above wouldn't work under any version because it's missing parameter declarion. You can find correct declaration in ShellExecute API #12636. The function itself should work under any 32 bit Windows version.

>2- Will Shell32.DLL file be kept in newer versions of windows
>for compatibility?

So far MS didn't provide any Windows API functions that supercedes it. Even if they do, it will be avaialable for many versions.