How to check windows Forms Instance running or not ?

There are a couple of ways:
You could keep a reference to it in your current form class, and check that:

private MyForm myForm = null;
private void OpenNewMyFormInstance()
    {
    if (myForm == null)
        {
        myForm = new MyForm();
        myForm.FormClosed += new FormClosedEventHandler(myForm_FormClosed);
        myForm.Show();
        }
    }
void myForm_FormClosed(object sender, FormClosedEventArgs e)
    {
    myForm = null;
    }

You need to handle the FormClosed event to register that the user closed it.

Or, you could check for an existing instance:

bool isMyFormOpen = false;
foreach (Form f in Application.OpenForms)
    {
    if (f is MyForm)
        {
        isMyFormOpen = true;
        break;
        }
    }
if (!isMyFormOpen)
    {
    MyForm f = new MyForm();
    f.Show();
    }

Me? I'd keep the reference, because I might want to use it for something else.