Microsoft have made Vista even more stable with SP1 (though the major contributor is probably Vista ready drivers), and have at the same time given XP some love by collecting all patches released since SP2 into a single tested SP3 package.
Vista SP1 introduces a new Secure Socket Tunneling Protocol service, which allows VPN over SSL and makes it easier to setup VPN connections.
WinXP SP3 now also includes the Network Access Protection Agent, which makes it easier to protect the network against simple user mistakes, such as forgetting to apply security and antivirus updates. The Wireless Zero Configuration have been split into two, so now it only takes care of Wireless 802.1X authentication, while the new Wired AutoConfig service takes care of wired connections.
Note Firefox 3 RC1 also seem ready for prime time (besides the usual bug fixes). Been using FF3 the last few weeks and it has been very stable (Had to upgrade my firefox addons and the Flash Player plugin manually). Really like the new features in the address-bar, but I miss the persistent Go-button. Another new feature is the ability to duplicate a tab by holding down the CTRL-key while dragging the tab with the mouse (Would be nice with a right-click option). Detailed list of Changes in Firefox 3