How do I prevent someone from downloading and installing on my computer

The easiest way is to not let them use the computer.

Otherwise, setup another user account for them with reduced privileges. For example, within Microsoft Windows, this account would only be a member of the Users group. What software are you trying to prevent installation of? Some software like ClickOnce (http://en.wikipedia.org/wiki/ClickOnce) applications will install with only User rights.