If my machine has 3 users (say, Bill, Jane and Root) how much spill is there between users?

Some example scenarios:

1- If Bill installs gtk-gnutella, can Jane run it?
2- If Jane then uninstalls gtk-gnutella, can Bill no longer run it?
3- If Root installs gtk-gnutella, can Bill break it?
4- Jane really tries to muck everything up in her account (e.g. malicious deleting/renaming), is it possible to break Bill's account?
5- Bill logs into a new machine first (say, mandrake) and gets all the "welcome to mandrake, let's set up your email", does that stop Jane from getting the same, or do all new users get this greeting?
6-If Jane makes all her KDE windows pink and twiddles all the look'n'feel settings will that affect Bill?

My guessed answers:
1-Yes
2-I guess Jane wouldn't be able to uninstall an app that "belongs" to Bill.
3-?
4-Jane cannot break Bill's account
5-?
6-?

user software installs ->the user only can run it
root software installs ->any user can run it
Users can only use programs, not change them, that's root's job.

General rule: users have access to their account only and to others as the system administrator allows by adding them to certain groups. If you don't put a user in the wheel group or give the user root's password, they can't mess with each other's stuff and can't mess with the system if it is set up properly.

LOL at "Cold Guinness, the geek way" in your sig!