isAdmin function and administrator rights in Windows 7

I have programmed a Delphi application (I am using Delphi 2006 ) that checks for icons associations at startup. For this reason I have been using the isAdmin function that I found on the web. I have attached a simplified piece of my code; in brief in the FormShow procedure I pass

      if isAdmin then GetAssociations;

and then in 'procedure GetAssociations' I open a Registry key and read and write to it.

In Windows Xp and Windows Vista the program faced no difficuties and, whenever the user had administrator rights, it associated the icons.

On the contrary, in Windows 7 when the user is an administrator the program moves to the GetAssociations procedure (i.e. isAdmin=true), and opens the registry key and reads successfully from it. However, it cannot write to the registry and when I try to WriteString to the Registry, it gives a glorious error message: Failed to set data for ''.
Note that when I right-click on the executable and I Run as Administrator the error does not occur.

Any ideas of how to solve this problem in Windows 7?

Thanks in advance,

procedure TMainForm.FormShow(Sender: TObject);
begin
    //associate the AppIcon to the input files
    if isAdmin then
        AssociateIcons;
end;

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 TMainForm.AssociateIcons;
var
  str : string;
  AskNow,AskNext : boolean;
  Assoc : boolean;
begin
    AskNow := RIniFile.ReadBool
                   ('Settings\General','AskAgainIcons',true);
    AskNext := AskNow;
    with TRegistry.Create do
    begin
        Assoc := true;
        RootKey := HKEY_CLASSES_ROOT;
        //first for project files
        OpenKey('\'+AppShortName+'.input',True);
        WriteString('',AppShortName+' Project File');
        //.................
        //...more code...
        //.................
        
        Destroy;
    end;

     RIniFile.WriteBool
            ('Settings\General','AskAgainIcons',AskNext);

end;

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