I hate how people use the statement "Oh, how do you know your computer isn't infected with a whole bunch of stuff if you don't scan" Simple answer, if you KNOW computers, and KNOW what your doing, and something manages to get onto your computer, you will notice it.
For instance, if my computer is being a little sluggish (I only have one 256MB ram chip installed) I open up the process window of Task Manager, so if something was there I would see it. I'm familer with my startup list, as is any computer geek. I know sketchy windows folders from legit ones. But if I wasn't the only one using this computer. I would throw on some AVG Free just incase.
~Bryan~