You must extract or unzip the file to your themes folder which can be 
found - program files/plus/themes.

To install -

either right click anywhere on your desktop and click on properties 
and then click the "themes" tab.  Then click the name of the theme you 
wanted to install from the pop down menu and click "apply"

or

go to your control panel (start/settings/control panel) and click on 
"Desktop Themes" and choose the theme you want to install from the pop 
down menu. 


Note - 

If there is something in the theme you don't want to install then 
just uncheck the relevant box before clicking apply.