will anybody help me with this..I copied the code below from Microsoft Windows Sharepoint Services SDK but it just wont work everytime i execute that code. can anybody body please tell me whats wrong or is there something i missed in that code..by the way the error in the code below happens in this line

Code:
System.Xml.XmlNode nodeListItems = listService.GetListItems(listName,viewName,query,viewFields,rowLimit,queryOptions);
Code:
            // Declare and initialize a variable for the Lists Web Service.
            List_Service.Lists listService = new List_Service.Lists();

            /* Authenticate the current user by passing their default 
            credentials to the Web Service from the system credential cache.*/
            listService.Credentials = System.Net.CredentialCache.DefaultCredentials;

            // Set the Url property of the service for the path to a subsite.
            listService.Url = "http://sharepoint/_vti_bin/Lists.asmx";

            // Instantiate an XmlDocument object
            System.Xml.XmlDocument xmlDoc = new System.Xml.XmlDocument();

            /* Assign values to the string parameters of the GetListItems method, 
            using GUIDs for the listName and viewName variables*/
            string listName = "{17991794-81BB-494F-9910-CFBF1093A7CF}";
            string viewName = "{7137FFF8-48FF-4C69-8C76-0E3BBD1EA7F9}";
            string rowLimit = "150";

            /*Use the CreateElement method of the document object to create elements 
            for the parameters that use XML*/
            System.Xml.XmlElement query = xmlDoc.CreateElement("Query");
            System.Xml.XmlElement viewFields = xmlDoc.CreateElement("ViewFields");
            System.Xml.XmlElement queryOptions = xmlDoc.CreateElement("QueryOptions");

            /*To specify values for the parameter elements (optional), assign CAML fragments
            to the InnerXml property of each element*/
            query.InnerXml = "<Where><Gt><FieldRef Name=\"ID\" />" +
            "<Value Type=\"Counter\">3</Value></Gt></Where>";
            viewFields.InnerXml = "<FieldRef Name=\"Title\" />";
            queryOptions.InnerXml = "";

            /* Declare an XmlNode object and initialize it with the XML response from 
            the GetListItems method.*/
            System.Xml.XmlNode nodeListItems = listService.GetListItems(listName,viewName,query,viewFields,rowLimit,queryOptions);
            
            // Loop through each node in the XML response and display each item.
            foreach (System.Xml.XmlNode listItem in nodeListItems)
            {
                label1.Text += listItem.OuterXml;
            }
Error details...

System.Web.Services.Protocols.SoapException was unhandled
Message="Exception of type Microsoft.SharePoint.SoapServer.SoapServerException was thrown."
Source="System.Web.Services"
Actor=""
Lang=""
Node=""
Role=""
StackTrace:
at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
at System.Web.Services.Protocols.SoapHttpClientProtocol.Invoke(String methodName, Object[] parameters)
at WindowsApplication1.List_Service.Lists.GetListItems(String listName, String viewName, XmlNode query, XmlNode viewFields, String rowLimit, XmlNode queryOptions) in C:\Documents and Settings\absalvamante\My Documents\Visual Studio 2005\Projects\BCMD\WindowsApplication1\Web References\List_Service\Reference.cs:line 484
at WindowsApplication1.frmindex.button2_Click(Object sender, EventArgs e) in C:\Documents and Settings\absalvamante\My Documents\Visual Studio 2005\Projects\BCMD\WindowsApplication1\frmindex.cs:line 778
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.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam)
at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoCompo nentManager.FPushMessageLoop(Int32 dwComponentID, Int32 reason, Int32 pvLoopData)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context)
at System.Windows.Forms.Application.Run(Form mainForm)
at WindowsApplication1.Program.Main() in C:\Documents and Settings\absalvamante\My Documents\Visual Studio 2005\Projects\BCMD\WindowsApplication1\Program.cs:line 17
at System.AppDomain.nExecuteAssembly(Assembly assembly, String[] args)
at System.AppDomain.ExecuteAssembly(String assemblyFile, Evidence assemblySecurity, String[] args)
at Microsoft.VisualStudio.HostingProcess.HostProc.RunUsersAssembly()
at System.Threading.ThreadHelper.ThreadStart_Context(Object state)
at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state)
at System.Threading.ThreadHelper.ThreadStart()