There are no viruses for OS X however there is some malware. I'd recommend carefully reading Thomas Reed Malware Guide for more information. You are correct about Norton and as a general rule it is not wise to run AV applications on your Mac as they tend to create more problems than they solve.
Your problem is probably due to you using the same password for your email as other applciaitons. I'd recommend immediately changing your password(s) to something more cryptic such as "Ktt87&^b" which would be very difficult to figure out.
Bottom line really is the best thing you can do to protect your Mac from Malware is to run Software Update very frequently.
It could be that a phishing scam got a hold of your contacts email addresses via facebook or some other social site means. Sometimes these scams can go through facebook and myspace. It does not necessarily mean you have a virus on your mac. I would contact the friends that your receiving them from and see if they are experiencing the same thing.