One of my hobbies is to tweak the GUI (graphical user interface) of computers, to make them more appealing. Below you can see screenshots of some of my favorite themes, click for full size view. If interested, you can patch a file called uxtheme.dll in Windows XP (located in C:\Windows\system32) to enable use of third party visual styles. To do this you must use a program called replacer, and a patched version of the uxtheme.dll file. For a great collection of Windows visual styles, check out deviantART. For the widgets on the side I use the popular Yahoo Widgets program. For some awesome wallpapers, check out interfacelift. Happy themeing!
Windows XP (mimicking Vista):