My dad's Hotmail box got filled with EMBARASSING spam, I don't know why, and the problem was, the senders were always different, so you couldn't block them!
I don't know why. My father is really clumsy at using Internet (actually it was me who checked his mail) and I don't think he gave his address in some strange sites or so.
I had to change his account, but mine works OK, I get really few spam. I think the most mess comes out when you answer a spam message: this way they're sure your account is working, and they probably tell other spammers! So let's remember we must not answer any spam stuff.
And if you have to give your address to people you don't trust very much, like sex sites, it's better to have another less important account for things like these.