I use Hotmail for a main email account. They generally have good spam filtering on the incoming email. Unfortunately they don't do so well on outgoing.
"I" ended up sending spam. Out to my whole email list. I don't know how it happened, since I am quite careful and well aware of various net pitfalls.
I suspect that my son browsed to an unsafe site while I had an open Hotmail window. I was using Internet Explorer of course. Thank you, Bill Gates!
I ran a virus check (definitions were only 24H out of date on my previous scan less than 12 hours before), nonetheless I had to check. No virus found. Also changed my Hotmail password from a wired computer in case of wireless intercept. I use 128bit WEP, have IP-selective service, and my nearby neighbors are idiots by all indications, so a wireless intercept is unlikely. Still, you can't be too cautious.
I had used IE because certain people in my house don't seem to get Firefox with Noscript. Now they will have to learn to love it.