If you’ve started to get bored with the general Windows XP desktop then the following might be for you.
Step 1 - Replace the desktop theme
Microsoft has put out a really sharp-looking XP desktop theme based on Windows Media Player 11. It simply installs as another desktop theme in your Display Options; no need for any third-party applications.

Update! You might also want to try out Royale Remixed by “~oddbasket”. Less bugs and and a very beautiful theme indeed. Thanks to Balu for sharing!
Step 2 - Add a new background image
If you haven’t replaced the background image already you could visit Deziner Folio and get the set of 74 Hi Res Windows Vista Wallpapers.

Step 3 - Live it up a little
Install Google Desktop and add live search and all the gadgets you can fit on your desktop. After a minute or two your Windows XP could look something like this:

There you go, 3 simple steps to make your welcome screen less boring. Feel free to add more steps through the comments below if you know of any other eye-candy’s worth having!