E-mail, chat, & VoIP : my email address is being used for spam

my email address is being used for spam

Apparently my email address is being used to send spam. There have been two email that have been bounced back to me because they could not be delivered, however, I never sent them. I have a second email account which is listed in my address book and have not recieved any spam there so, as far as I know, the spam is not going out to the people in my address book. I have Verizon dsl and I use Netscape 7.2 for my browser. I use two different computers to go on line - one has WindowsXP the other has Windows 98. I have Verizon dsl and use Netscape 7.2 as my browser and mail reader on both computers, although I am no longer downloading or sending mail on the Windows 98 desktop. The XP computer has ZoneAlarm Internet Security Suite 2006. The 98 computer has Norton anti-virus.

I suppose there are two possibilities. Either a spammer is simply using my email address as a return address or one of my computers has been hijacked into sending out the spam. What can I do now?

I presume that there are two possibilities