where do I start?...

Code:
 MessageBox.Show(ex->Message + "\n\n" + 
                         ex->GetType()->ToString() + "\n" +
                         ex->StackTrace, "Exception");
"->" is not valid in C#. the code should be this..

Code:
MessageBox.Show(ex.Message + "\n\n" + 
                           ex.GetType().ToString() + "\n" +
                           ex.StackTrace, "Exception");
==========================================

Code:
If   (IsValidData());
C# is a case-sensitive language. There is no "If"(capital "I") statement, but there is an "if"(lowercase "i") statement.

You also shouldn't have the semicolon at the end of the if statement. That is basically terminating the if statement.

Code:
if (IsValidData())
{
    // your code
}
=============================================

Code:
txtOverallNPV.Text=npv.tostring();
txtOverallROI.Text=roi.tostring();
txtBrkEven.Text=bep.tostring();
These should be ".ToString()".

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I didn't go through the rest of the code.