Question about Trojan Horse

I am running Windows XP on my computer at home - and have created different accounts (those with Windows, know what I am talking about) - one for me, one for my wife, and one for my kid.

I am pretty careful what I am downloading, and which places to go to. In addition, I have three anti-spyware programs running, plus subscribe Norton Commander and there is another free software that provides firewall services (I forgot the name, but it is highly recommended by services, such as Download.com). About once a week, I am running those programs to get rid of the possible nasties on my computer - if there are any, but also of cookies etc.

However, yesterday, I was in quite for a shock. While my account was free of anything, besides some cookies, my kid's account had a Trojan Horse - he is sevn years old, and he doesn't access the computer alone - so he didn't download anything.

My question to those experts amongst the Malaysian bloggers is:

- How can it be that my kid's account has a Trojan Horse, when my account didn't even indicate such nasty

- More important: Would you recommend that I change all my passwords?

Your comments are appreciated!!