UPDATE: September 29, 2008
This post is relatively old and at that time, this was the only theme available. Currently there is a Zune theme for Windows XP which is pretty cool and gives the Vista Look n Feel.
I would recommend not going for any third party "Vista look for XP" since they consume a lot of memory resources as well as result in leakages / spyware.
The same can be downloaded from here
Ever had a chance to look at the Windows Vista ("Longhorn") Screens? Waiting to explore the rich graphics based on Windows Presentation Foundation ("Avalon")?
For those who are itching their hands to have the new, fresh UI look for your Desktops, there is already a similar theme available for your Windows XP.
It is known as "Energy Blue" theme and it gives a fresh new look for your Windows XP, quite similar to the rich graphical interface to be shipped with Windows Vista.
The Energy Blue theme can be downloaded for free from here.
After downloading and installing the same, change your Desktop Themes Settings to "Energy Blue" to get the fresh new look.
This would provide your desktop a fresh look for some time until Vista ships.
Cheers !!!