باید از کد زیر استفاده کنی
System.Threading.Thread.CurrentThread.ApartmentSta te = System.Threading.ApartmentState.Unknown;
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(fals e);
Forms.FrmLogo Frm_t;
Thread t_Show_Form = new Thread(delegate()
{
Frm_t = new Forms.FrmLogo();
Frm_t.ShowDialog();
});
t_Show_Form.Start();
//عملیات محاسباتی
t_Show_Form.Abort();
Forms.FrmLogin f = new Forms.FrmLogin();
if (f.ShowDialog() == DialogResult.OK)
{
Application.Run(new FrmMain());
}