There's no doubt that the freeware Tweak UI is a tool designed for people comfortable and confident with mucking about in the guts of their Windows XP installation. If that's not you, we strongly caution against using this program capriciously.
However, if you're the kind of person who likes to get your hands dirty with XP, read on. Tweak UI can alter most of the routine behaviors of Windows XP and Windows Server 2003, but it also can affect some of the more advanced ones, too. The basic interface belies the features, from customizing the Alt-Tab program switcher to removing the white arrow that appears on shortcut icons, from killing the autoplay feature for specific drives to configuring file and print sharing settings. Tweak UI is a must-have.