Use the following simple function ..
E.g. like this:
SetSystemEnvironmentVariable ('PATH', 'c:\windows;c:\programs\myapp');
procedure SetSystemEnvironmentVariable (const name, value : string);
var
rv : DWORD;
begin
with TRegistry.Create do
try
RootKey := HKEY_LOCAL_MACHINE;
OpenKey ('SYSTEM\CurrentControlSet\Control\SessionManager\Environment', False);
WriteExpandString (name, value);
SendMessageTimeout (HWND_BROADCAST, WM_SETTINGCHANGE, 0,
LParam (PChar ('Environment')),
SMTO_ABORTIFHUNG, 5000, rv);
finally
Free
end
end;