Some users will have upgraded to Windows Vista, but prefer the look and feel of Windows XP. There are two options depending on which XP look you prefer (Classic or Default).
If you prefer the Classic XP layout, you can revert Vista back to this quite easily. First, right click on your desktop and click Personalize:
Now, select Windows Classic from the drop-down box and click OK:
You should now be viewing Vista in the classic XP theme (how Windows 95/98/2000 look).
Default XP Theme
If you prefer the more colorful XP theme you will need to install a custom theme to mimic this.
Some examples of custom XP themes are:
Each theme will have it's own instructions to follow, but they are straight forward to install.