I agree FireFox would be a good start as it can stop scripts, cookies, files, etc from being downloaded or run on your computer.
To create your multi-OS environment you will have to make the 4 partitions, but you will need to ensure your family does not play around with partitions outside of there own, unless you want to actually lock out other drives.
http://www.syschat.com/dual-boot-vis...eady-1946.html
This tutorial will help you get XP installed, without destroying your Vista installation. Once that is completed, install your Linux, and ALLOW the grub bootloader to be used, Vista is quite compatible with it, when it boots you will be given grub options, if Windows is selected, you will then be given the list of Windows OSs.