[marcus]

Installing an app on Vista/XP almost always requires administrative privileges even if the only need stems from writing into the C:\program files\ directory.

And frankly once you give an application Admin privileges once, it doesn't matter anymore it can do whatever the hell it wants. Sandboxing is a much better mechanism from a security perspective

Plus uninstalling a program is always problematic under the existing Windows mechanisms, basically it is every programmers duty to write an uninstaller for his own software, many of which are buggy, leave trash on the computer and so on. Sandboxing an installation makes the uninstall automatically easy & fool proof.