Shoutbox

I'm back form my holidays and I have noticed that someone has been  (intentionally) sending viruses to others using my address as sender (this is possible and even easy changing some parametric information in the e-mail).

This person has even got to register him- or herself on boards using my username and e-mail address.

As you will understand, this is certainly annoying.

Are there any way to execute countermeasures for this kind of hijacking?
Knowing the IP won't help much, sadly, but there's maybe a flaw in the way mails have been sent or... well, who knows.

I'd be very pleased if someone can provide any information about this issue...

Thanks