Microsoft has published a document on the Microsoft Developer Network (MSDN), "How to Make Your Web Site Work with SP2." The article covers design changes you might need to consider regarding ActiveX controls, file download mechanisms, pop-up windows, Java, HTML dialogs, and window positioning restrictions.
For example, if your site automatically (through scripting) opens more than one pop-up screen per Web page, you'll need to modify it so that no more than one pop-up screen appears because Windows XP Service Pack 2 (SP2) will cause Microsoft Internet Explorer (IE) to disallow more than one pop-up screen unless the user clicks an object to cause such a pop-up to open. And if other objects (such as Flash) try to open a pop-up window, that too will be disallowed, so developers will need to consider changing that behavior.
The new way that XP SP2 handles downloads might affects lots of sites. If a download is initiated through a method that isn't user initiated (such as automatic Web page redirection), IE will block the download.
Content types must also match the content; if they don't match, the content might not appear properly. For example, if an HTML document has its content type set to "text/plain" then the document won't be rendered as HTML.
For more details about other potential design issues be sure to read the document at MSDN.