In ASP.NET 2.0, you can use the System.Configuration classes, see
http://www.codeproject.com/KB/dotnet/mysteriesofconfiguration.aspx

You can also write to the web.config file in the same way as you would to
any XML file, something like (in VB.NET):
-----------------------------
Try
   ' Open web.config file
   Dim doc As New XmlDocument()
   doc.Load(Server.MapPath("web.config"))

   Dim strSel As String
   ' Use an XPath query to look up the user element in this configuration
having
   ' a matching "name" attribute
   strSel =
"/configuration/system.web/authentication/forms/credentials/user[@name='" &
strUserName & "']"
   Dim node As XmlNode = doc.SelectSingleNode(strSel)
   ' Modify the element
   Dim element As XmlElement = CType(node,XmlElement)
   element.SetAttribute("password",strHash)

   ' Save the configuration
   doc.Save(Server.MapPath("web.config"))
Catch ex As Exception
   Trace.Warn(ex.ToString())
End Try
------------------------------
Keep in mind that changing web.config may cause your application to restart,
thereby losing the session information for all users.

Is this any use http://odetocode.com/Articles/418.aspx

Regards,
Mark

The way to go is to put all your setting that can change in a separate file.

Put this line in the <configuration> section on web.config

<appSettings file="MySettings.xml"/>

Having done that, you can access parameteres in MySettings.xml via
ConfigurationManager.AppSettings.