My family uses AIM or Yahoo Messenger quite often to talk to each other, or open a room for us to all talk together.  

I have one grown son off on his own, two in different colleges, and I am sometimes away due to work.  

AIM, Yahoo, and ICQ are quite common IM programs.

It IS possible for them to get worms, viruses, or spyware through these programs.  So, you need to talk to them about such things.  No downloading "search bars" or other "free" utlities or other cute things ("Comet Cursors") without talking it over with you first.  And YOU need to educate yourself on such things.

You will need to go into each of these programs and set them to not open on bootup, not conntect automatically.  And instruct your kids to turn these programs off when they leave the computer.

Also, they are not to download games unless they speak with you first.  "Free movies"... no, they are not to click on any of that kind of thing.

Again, you need a good firewall program as I mentioned before.

And even with all these precautions, be prepared, don't hit the ceiling, when you get up one morning and your home page is some porno site.  It happens.  You will have to learn how to remove these things.  Don't yell or blame the kids... one of my coworkers got one of these trojans on his work issued computer while looking at a farm equipment auction site.  This is just something we have to live with these days.

Buy an external harddrive for backup, and use it often.