Virus scanners slow down.. I just uninstalled Norton SystemWorks 2003 Professional, since my PC performance was just not acceptable.
My startup time took 16 seconds LONGER! My 3D Mark score decreased with 30%! In 3D games, my framerate is 50 FPS lower! Those are all reasons why I don't use it anymore. Sure, it's safe, but if you act carefully, and don't click yes on those annoying .. pop-up's everything should be fine.
I also don't open files in e-mail attachments. If I want a file from somebody, I use an encrypted compressed folder, containing the document/file. This might slow stuff down, but it does work, since you can inmidiatly see weither or not the attachment is really personal.
Another thing, I get those really
chain emails from some friends. With all of there e-mail addresses in the CC bar! That's a spam walhalla! I could actually get around 500 e-mail addresses from 1 email.. Its insane. The content in even worse. Its got a fake Microsoft mail saying your hotmail (do I use hotmail? NO!) account will be deleted if you don't forward it to at least 10 friends. Just by the design of the mail you can see this whole thing is a big joke. I always use the 'Reply-to-all' function and say some stuff.
Or those other letters, saying you're life will fail if you DONT forward this mail, or if you send it to less then 10 friends. From 10-20 you'll live happy, 150+ and you're life will be GREAT! You'll always have money enough, minibars everywhere, etc etc. 
Makes me see that some human beings aren't quite as sophisticated as I thought. 