Andrea you could set up your kids limited user accounts in fact each could have their own if you wanted it that way, but none should have administrator accounts that will keep anything from "accidentally" getting installed by the kiddos.
Backing up is very important. There are very good imaging programs now that let you create an image of your entire system which is an excellent idea. Doing multiple methods of back up is the best way to go, back up to cd or dvd, flash drive or usb hard drive, and create an image. I also use some online storage sites to store some of my important files.
I use zonealarm pro but one of the older versions, I hate it that they have started adding that pre-checked toobar, they should be ashamed.