Pay attention to the stuff you install. Install only one program at a time. Don't let your kids install anything unless they fill out the log. Make sure you're able to get rid of a program completely if it causes a problem after you or your kids install it. This means removing all traces of the program and restoring the entire computer system to the state it was in before the errant software was installed.
Backups are a pain, but so is going to the dentist.
There are times when you need to do both. Backing up everything is overkill, because you can restore many programs by reinstalling them from the original CDs or by getting replacements for shareware programs off the Internet. But you're playing with disaster if you don't have current backup copies of the files that tell Windows how to do everything. Making those backup copies isn't hard. Next week I'll explain how to do it, step by step.