|Per-User ("Just Me") Installations|
Per user resources are those that only effect a single user, that is "HKCU" registry and profile directories for file (no "Program files"). See "system folder properties" for locations Windows Installer defines along with some sample values.
The user may or may not have permissions to any other resources and assumptions about permissions to such objects are a constant source of issues and may mean that an administrator is required to install a product where not really required (more of a problem under "Vista").
Under Vista if you don't access per-machine resources you still have to prevent user elevation (LUA) by setting the "COMPANY_PACKAGE_REQUIRES_ELEVATED_PRIVLEDGES" macro to "N".
Per-User installs will only appear in the Add/Remove Applet if the product was installed for that user (the one running the applet).
Nothing prevents a per user installation modifying per-machine resources but this should be avoided whereever avoided and tested well with multipe users to ensure these resources are shared correctly.