Rule number one
Always protect your user with a good password and never, NEVER, leave the default Administrator's password blank.
This is one of the most common issue, a real red carpet to any worm virus that attempts to infect your system from the network.
Would you live in a house without a mean to lock your front door? No, because you don't want anyone to get in if it isn't you that allow them in and your adminisitrator's password acts as a lock.Sadly enough, untill Vista and, if i remember correctly, XP service pack 2, all the Microsoft operating systems allowed to install the OS while leaving the admin password blank.
Don't believe me? Try it, check the random XP computer and see if the admin's password isn't blank.. chances are that it will be, because most end users just click 'next' without reading much.
Of course, you must protect any user with a password.
Rule number two
If you don't know it, don't run it.
Looks obvious, but this is another of those things that most people doesn't pay attention to.
It's often that funny thing that a friend, often innocently, sent you that is infected by something.
Never trust mail attachments, unless you're pretty sure that who sent the mail actually meant to send it.
Never trust links from emails you don't recognize, it's quite easy to disguise a link in an email and you might be clicking on a link that is not what you think it is.
I, for one, delete any mail whose sender i don't know and whose subject doesn't make any sense to me.
I probably end up deleting one good email in a hundred but at least i reduce risks by 100%, which i prefer.
Rule number three
Turn on automatic updates, keep your system always updated.
Worms and viruses often take advantage of security holes of your operating system, old security holes for which an hotfix usually is released long time before any bad guy starts using them to their advantage.
Basically an updated system is a lot more hard to infect than one that never gets updated.
to be continued..