Vista: Right-click the desktop background and select ΓÇ£personalizeΓÇ¥. Click ΓÇ£Screen SaverΓÇ¥, then select the screen saver from the drop down menu.
XP: Right-click the desktop background and select ΓÇ£PropertiesΓÇ¥. Click the ΓÇ£Screen SaverΓÇ¥ tab and then select the screen saver from the drop down menu.