Null Reference Exception

Question

HI, I have a simple application, which use to run on Win Xp, Visual Studio .Net 2003 I moved the application to a new box, which has Win 2000,Visual Studio .Net 2003 my code has a string declaration such as string mystring = "This is a test"; this line used to work on the older box, but it throws a NullRefrenceException, at certain portions of my code. It works in certain procedures and does not in others. What should i do to get rid of this problem. Thanks -Saral Shodhan