Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

const
   SECURITY_NT_AUTHORITY: TSIDIdentifierAuthority =
     (Value: (0, 0, 0, 0, 0, 5));
   SECURITY_BUILTIN_DOMAIN_RID = $00000020;
   DOMAIN_ALIAS_RID_ADMINS = $00000220;

 function IsAdmin: Boolean;
 var
   hAccessToken: THandle;
   ptgGroups: PTokenGroups;
   dwInfoBufferSize: DWORD;
   psidAdministrators: PSID;
   x: Integer;
   bSuccess: BOOL;
 begin
   Result   := False;
   bSuccess := OpenThreadToken(GetCurrentThread, TOKEN_QUERY, True,
     hAccessToken);
   if not bSuccess then
   begin
     if GetLastError = ERROR_NO_TOKEN then
       bSuccess := OpenProcessToken(GetCurrentProcess, TOKEN_QUERY,
         hAccessToken);
   end;
   if bSuccess then
   begin
     GetMem(ptgGroups, 1024);
     bSuccess := GetTokenInformation(hAccessToken, TokenGroups,
       ptgGroups, 1024, dwInfoBufferSize);
     CloseHandle(hAccessToken);
     if bSuccess then
     begin
       AllocateAndInitializeSid(SECURITY_NT_AUTHORITY, 2,
         SECURITY_BUILTIN_DOMAIN_RID, DOMAIN_ALIAS_RID_ADMINS,
         0, 0, 0, 0, 0, 0, psidAdministrators);
       {$R-}
       for x := 0 to ptgGroups.GroupCount - 1 do
         if EqualSid(psidAdministrators, ptgGroups.Groups[x].Sid) then
         begin
           Result := True;
           Break;
         end;
       {$R+}
       FreeSid(psidAdministrators);
     end;
     FreeMem(ptgGroups);
   end;
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 begin
   if isAdmin then
     ShowMessage('Logged in as Administrator');
 end;