How to Set Welcome Page Programmatically in SharePoint?

By default, SharePoint uses "default.aspx" page as the welcome page (or Home page) in SharePoint 2007 and "SitePages/Home.aspx" in SharePoint 2010. It uses Pages/Home.aspx when publishing feature is enabled under MOSS 2007.

If you want to change Home Page: SharePoint provides users an interface to set the welcome page once Publishing feature is enabled. (By going to Site actions >> Site Settings >> Welcome Page)

and specify the welcome page.

What if You don't want to enable the publishing feature, but want to change the Welcome page? Just hit this URL in browser:

http://Sharepoint-site-URL/_layouts/AreaWelcomePage.aspx  ,change the Sharepoint-site-URL accordingly.

Want to do it Programmatically? Yes, We can set welcome page in SharePoint 2010 programmatically with Object Model. Here is how:
Change welcome page programmatically Using C# Object Model:

using (SPSite site = new SPSite("http://sharepoint.com")) 
{
    using (SPWeb web = site.RootWeb) 
    {
        SPFolder rootFolder = web.RootFolder;
        rootFolder.WelcomePage = "Pages/HomePage.aspx";
        rootFolder.Update();
    }
}

Set Welcome Page in SharePoint Using PowerShell:

$SiteUrl = "http://sharepoint.com"
$Site = Get-SPWeb -identity $SiteUrl
$RootFolder = $Site.RootFolder;
$RootFolder.WelcomePage = "SitePages/HomePage.aspx";
$RootFolder.Update();
$Site.Dispose()