There are several ways to install presets, depending on which kind they are. For Develop Presets, the easiest way is to switch to the Develop Module [D], right-click the User Presets subfolder, and select Import…. Navigate to your presets and Import them.
To manually locate presets, use the following locations:
C:\Documents and Settings\<username>\Application Data\Adobe\Lightroom\
The various Preset folders are located in subdirectories below these.