C#

if (txtdisc.Text == null)
{
    ..
    Double txtdisc22013 = Convert.ToDouble(txtdisc.Text);

Double txtdisc22013 = Convert.ToDouble(null);

 if(string.IsNullorEmpty(txtdic.Text) != true)
 {
   //...
 }

or 

 if(txtdisc.Text != string.Empty)
 {
  //...
 }

Hi
First, like others mentioned, "if" and "else" are conditional statements, they are not loop.
Second, the textbox will return empty if it has no value assigned to it, so checking for null is redundant.
Third, this is a coding horror!

if (txtdisc.Text ==null)  This check is redundant therefore  the calls below will never be executed.
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   Double txtdisc22013 = Convert.ToDouble(txtdisc.Text); Converting a null to a nullable Double will 0.0;

   txttot22013 = txttot22013 - txtdisc22013; (Some value – 0.0)

   txttot.Text = txttot22013.ToString();
}
else
{
   txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;

   txttot.Text = txttot22013.ToString(); The value of "txttot22013" in the else conditions  is no different to the one in if condition.
}

if (txtvat.Text == null)
{
   MessageBox.Show("Enter Vat Amount"); This will never be called.
}
else
{
   Double txtvat22013 = Convert.ToDouble(txtvat.Text); Possible exception.

   txtvat22013 = txtvat22013 / 100;

   txtvat22013 = txtvat22013 * txttot22013;

   Double txtvatinclu22013 = txtvat22013 + txttot22013;

   txtvatinclu.Text = txtvatinclu22013.ToString();
}


If I rewrite your code, it may as follows:

txttot22013 = txtpr22013 + txtpr22014 + txtpr22015 + txtpr22016 + txtpr22017;
txttot.Text = txttot22013.ToString();

if (string.IsNullOrEmpty(txtvat.Text))
{
    MessageBox.Show("Enter Vat Amount");
}
else
{
    double txtvat22013;
    if (Double.TryParse(txtvat.Text, txtvat22013))
    {
        Double txtvatinclu22013 = (txtvat22013 * txttot22013 / 100) + txttot22013;

        txtvatinclu.Text = txtvatinclu22013.ToString();
    }else
    {
    MessageBox.Show("Enter Valid Vat Amount");
    }
}