Microsoft began to talk about the upcoming Service Pack 2 for Windows XP at its recent There was a time when service packs were mostly just bunches of individual patches consolidated into a single program. Sure, there were new features in them but they were subtle features, like a new tab on a dialog box.
Windows XP has had an odd history with service packs. SP1, in addition to the usual consolidation of patches, added the court-ordered Set Program Access And Defaults applet, which allowed anyone to switch their Web browser, instant messaging and media player defaults. Of course, this gave competitors an even playing field and now Internet Explorer has vibrant competition (ha-ha!).
On the other hand, Service Pack 2 will make significant and noticeable differences in the behavior of the operating system in the interest of locking it down for security purposes. The changes—as they stand now many months before the release of SP2— are detailed in a paper on Microsoft’s developer site, MSDN.