You can customize the portal site pages and the pages of SharePoint sites created in the portal site just as you customize any other SharePoint site. Customization options include:
To customize SharePoint sites, you must have the appropriate rights. For more information about customizing SharePoint sites, see Customize your site.
You can also modify the portal site pages, including personal sites, by modifying the Web Parts found on that page.
As a member of the Administrator site group for the portal site, you can customize the Web Parts on portal site pages to change the default appearance of the portal site. You can also change the properties of Web Parts to prevent users from removing a Web Part from their view of portal pages or their personal site. For more information, see Modify shared Web Parts and Modify pages on My Site.
You can create custom enterprise applications that are integrated with Microsoft SharePoint Portal Server by using Enterprise Application Integration. For more information, see the Microsoft SharePoint Products and Technologies 2003 Software Development Kit.