Ho creato un software che presenta questo errore se eseguita su una macchina con xp sp3 (sulle altre funziona con win7 e xp sp3)

codice:
See the end of this message for details on invoking 
just-in-time (JIT) debugging instead of this dialog box. 

************** Exception Text ************** 
System.NullReferenceException: Object reference not set to an instance of an object. 
   at Rete.Form1.traceroot() 
   at Rete.Form1.button1_Click(Object sender, EventArgs e) 
   at System.Windows.Forms.Control.OnClick(EventArgs e) 
   at System.Windows.Forms.Button.OnClick(EventArgs e) 
   at System.Windows.Forms.Button.OnMouseUp(MouseEventArgs mevent) 
   at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) 
   at System.Windows.Forms.Control.WndProc(Message& m) 
   at System.Windows.Forms.ButtonBase.WndProc(Message& m) 
   at System.Windows.Forms.Button.WndProc(Message& m) 
   at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) 
   at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) 
   at System.Windows.Forms.NativeWindow.Callback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) 


************** Loaded Assemblies ************** 
mscorlib 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3603 (GDR.050727-3600) 
    CodeBase: file:///c:/WINDOWS/Microsoft.NET/Framework/v2.0.50727/mscorlib.dll 
---------------------------------------- 
Rete 
    Assembly Version: 1.0.0.0 
    Win32 Version: 1.0.0.0 
    CodeBase: file:///C:/Downloads/Software/Test%20Rete.exe 
---------------------------------------- 
System.Windows.Forms 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3053 (netfxsp.050727-3000) 
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Windows.Forms/2.0.0.0__b77a5c561934e089/System.Windows.Forms.dll 
---------------------------------------- 
System 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3053 (netfxsp.050727-3000) 
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System/2.0.0.0__b77a5c561934e089/System.dll 
---------------------------------------- 
System.Drawing 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3053 (netfxsp.050727-3000) 
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Drawing/2.0.0.0__b03f5f7f11d50a3a/System.Drawing.dll 
---------------------------------------- 
System.Configuration 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3053 (netfxsp.050727-3000) 
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Configuration/2.0.0.0__b03f5f7f11d50a3a/System.Configuration.dll 
---------------------------------------- 
System.Xml 
    Assembly Version: 2.0.0.0 
    Win32 Version: 2.0.50727.3082 (QFE.050727-3000) 
    CodeBase: file:///C:/WINDOWS/assembly/GAC_MSIL/System.Xml/2.0.0.0__b77a5c561934e089/System.Xml.dll 
---------------------------------------- 

************** JIT Debugging ************** 
To enable just-in-time (JIT) debugging, the .config file for this 
application or computer (machine.config) must have the 
jitDebugging value set in the system.windows.forms section. 
The application must also be compiled with debugging 
enabled. 

For example: 

<configuration> 
    <system.windows.forms jitDebugging="true" /> 
</configuration> 

When JIT debugging is enabled, any unhandled exception 
will be sent to the JIT debugger registered on the computer 
rather than be handled by this dialog box.
cosa può essere?? GRazie