Norton Firewall logging connections from usr/sbin/nmbd every 6 seconds... What is this, and how can I stop it?

Question

This whole situation first started with a complaint from my ISP that it appeared I had a trojan virus...  around 1100-1200 messages per hour were being run through their servers via my account.  I have also Anti-Virus enabled, so I was left scratching my head...
No viruses found on a full scan - so I started watching processes and connections.  This nmbd process is suspicious...  I don't run windows file sharing, nor have I ever.  This just popped up recently.  I also had two mac tech support calls, and one to Symantec - and it ran fine for a couple of days - but it's back again. 
What is this, and how can I find the culprit, and remove it permanently...?
Thanks in advance for any advice!
--Jeff

Answer 1

Thanks Thomas, appreciate the insight!  Thanks for taking the time to help me think through this...
I have reset the password  twice now...
It's only impacting one account, and the ISP says it's local to me - somewhere on my local network.
I do have a few devices on my home network.  The only one with windows is my macbook air running parallels.  I just use this to browse some web projects I work on (view in IE to make sure everything is looking like it should). 
The passwords I have used both times - they were ones set by my isp - the type you can't remember, they seem rather strong (upper/lower case letters, numbers, symbols).  That's what leads me to believe it's also local - something on my machine.  And it only seems to be impacting one email account (I have 5 running in Mac Mail).
WiFi network is protected by WPA2 - just checked to be sure.  All good there.
Now, in Norton Firewall log - I can see incoming and outgoing connections via Windows File Sharing/nmbd. 
The reason I feel/felt that this is related to the spam sends is that once I saw the number of connects, and roughly equals the number of sends per hour of spam - I stopped the process with the firewall and suddenly my isp says the spam sends stop. That led me to believe they are related. Perhaps this virus or malware has spoofed it's name and is identifying itself as nmbd?  I have no idea.  Just scared to turn it all off just yet.
I did notice that Moutain Lion does not run this...  (nmbd).
I did wonder about the Air sending something off of windows - but this all happened while it was off, laying on the desk next to me.  It rarely gets used unless I'm testing or traveling.
I can understand nmbd being useful part of the system, I cannot understand how it would be very useful if I didn't turn it on, it connects at that frequency, and I don't have file sharing enabled.  That's why I am hesitant to turn Norton off, and hope that everything just goes away.  I want to try and get this problem figured out as simply turning Norton off doesn't seem like I'm taking steps to eliminate the problem.  Perhaps Norton is causing other issues - and I'll be removing the software asap - but want to make sure the spam sends cease.
Let me know if that sparks any ideas...  Thanks again! 
--Jeff