basically, under windows, the users have authorisation to run programs by default, that have access to system files. also add to this the effect of things like outlook hiding file extensions and executing attachments by default, and someone can send you a file called something like britneystits.jpg.exe and not only will it look like a picture file to you in outlook, but outlook will notice that it is an exe and will run it without even asking you. so because you have access to all the system fiif this exe wants to mess them up, yhave given it permission to simply by reading your email.
suffice it to say that none of this happens with a linux system, because outlook isn't available for it, but more importantly because a user only has access to their own $HOME (this is called "My Documents" in mswindows) so if they try and run an executable file, firstly they know they are doing it, and secondly they can't change any system files, because linux's filesystems fully support permissioning (unlike windows's filesystems)
the main problem i see with this model is that you need to change to the superuser to install stuff, like rpm files or installable binaries for things like realplayer and java virtual machine. of course the superuser (i think called "Administrator" in mswindows) does have access to those system files, and this of course *could* create problems if the packages were malicious. This sort of thing has been more or less quashed by things like GPG signatures, verifying the source of the file, and also various switches for package managers like rpm, installpkg and so on, which allow you to see what changes will be made before you install. This is one benefit of having a specified package format, where a package to be installed is run by a seperate program, unlike under windows, where the package itself is runnable, and is not visibly installed by an installer program, and so there are no commands which can be used to determine whether the install will mess up your PC or not.
answer your question?