Kill family members?
If that is not an acceptable alternative, if the computer is YOURS, then I would make user accounts for each family member, assuming it is Windows XP or Vista, and only give yourself admin access. This will prevent other users from uninstalling software. If they are in Power Users default group, they may still install software on their OWN account as well as set up printers, and other users up to Power User access level, but they can only remove what they install, and they can't delete data in the folders of user accounts they did not create.
Also, in Windows XP, with admin access, when you install a program usually it gives an option to install for all users or "just for me" - but that means you still need individual user accounts.
-Skylark