Snowcounty

ParentSite = new SPSite(strUrl);

ParentWeb = ParentSite.OpenWeb();

ChildWeb = ParentWeb.Webs.Add(strChildName, strChildName, strChildName, (uint)1033, "SridharReflector.stp", false, false);

ChildWeb.Update();

ChildSite = new SPSite(ChildWeb.Url);

DChildWeb = ChildSite.OpenWeb();

SPNavigationNode ParentNode = new SPNavigationNode(ParentWeb.Title, ParentWeb.Url + "/default.aspx", true);

SPNavigationNode ChildNode = new SPNavigationNode(DChildWeb.Title, DChildWeb.Title + "/default.aspx");

ParentWeb.Navigation.TopNavigationBar.AddAsLast(ChildNode);

ChildWeb.Navigation.TopNavigationBar.AddAsFirst(ParentNode);

ChildWeb.Navigation.UseShared = true;

 Using a Custom Data Source for Navigation

http://msdn.microsoft.com/en-us/library/ms432695.aspx 

 Add link to the QuickLaunch menu using Object Model:

SPSite siteCollection = SPControl.GetContextSite(Context);
SPWeb site = siteCollection.AllWebs["TestSite"];
SPWeb subSite = site.Webs["SubTestSite"];

SPNavigationNodeCollection nodes = subSite.Navigation.QuickLaunch;
SPNavigationNode navNode = new SPNavigationNode("New Link", "http://www.msn.com", true);
nodes.AddAsFirst(navNode);