Get protected!
I keep getting around 300 virus emails/day to one of my accounts. I'm not pointing any fingers in any direction but I wish everyone would use at least some kind of protection on their computers. This is why that account get filled up sometimes.
I use Norton Internet Security myself and it is pretty good. I also use the free Microsoft AntiSpyware (beta though, but the best one there is).
For the uninformed: Some viruses and spywares make a little mailserver on your computer and start using addresses from your contacts and people you correspond with. They send out messages from your computer, using these addresses as the "from" address - basicly faking the sender. This is also why many get "undeliverable" messages from people they have never sent emails to.
Sadly, the "net" is what it is today and it is very unwise not to have up-to-date protection.