You have upgraded to Windows Vista and you want change the look and size of your desktop icons. There are two ways to do this.
You can make a right click on the desktop, and then select View.
A pop up window will appear with three options for Icons: Large, Medium and Classic.
The default icon size is on Medium so you just have to adjust it accordingly.
Large Icons are literally that. Large. They take up much space on the desktop.
The Classic view is the size that you are used to in XP.
The second and probably fastest way to adjust icon size is to select one of the desktop icons, then press the Ctrl key while moving the mouse wheel. This is also applicable in adjusting font size in other windows programs.