impersonation is not working for one computer

[memcom]

I have a ASP.NET 2.0 page that uses windows authenication and impersionation to login to my SQL2000 server. This works perfectly for about 10 different users on a number of different computers, but I have one computer that will not let me login to my website. The machine that is having a problem has WinXP Pro and is setup with much of the same software as mine. The error I am getting is login failed for user '(null)'. Reason: Not associated with a trusted SQL Server connection. I do not understand why the username is not being passed to the server. I have gone through Internet Explorer's options -> security and it looks like all the same options are selected as they are on the machines that work. I have set IE to prompt for a username and password and it still does not work. Does anyone have any idea what could be the problem? Below is my web.config file.

<?xml version="1.0"?>
<configuration xmlns="

http://schemas.microsoft.com/.NetConfiguration/v2.0">

<appSettings/>
<connectionStrings>
<add name="MyConnectionString" connectionString="Data Source=MyServerName;Initial Catalog=MyDatabase;Integrated Security=SSPI" providerName="System.Data.SqlClient"/>
</connectionStrings>
<system.web>
<compilation debug="true" strict="false" explicit="true"/>
<pages>
<namespaces>
<clear/>
<add namespace="System"/>
<add namespace="System.Collections"/>
<add namespace="System.Collections.Specialized"/>
<add namespace="System.Configuration"/>
<add namespace="System.Text"/>
<add namespace="System.Text.RegularExpressions"/>
<add namespace="System.Web"/>
<add namespace="System.Web.Caching"/>
<add namespace="System.Web.SessionState"/>
<add namespace="System.Web.Security"/>
<add namespace="System.Web.Profile"/>
<add namespace="System.Web.UI"/>
<add namespace="System.Web.UI.WebControls"/>
<add namespace="System.Web.UI.WebControls.WebParts"/>
<add namespace="System.Web.UI.HtmlControls"/>
</namespaces>
</pages>
<authentication mode="Windows"/>
<identity impersonate="true" />
</system.web>
</configuration>