There are new flavors of Mytob every day, sometimes every 1-2 
hours.  Norton releases updated definitions every week.  You can see where there's a window of opportunity there.
Mytob has lots of variants, and they have different behaviors.  Some install SDbot worms to make your computer take orders from outside and carry out criminal acts like participating in Distributed Denial Of Service attacks (DDoS), send Spam, and try to infect more computers.
Since you have Mytob and Norton 2004 isn't picking up on it, something's wrong.  There's a special class of viruses called "rootkits" that wedge themselves underneath Windows and filter what Windows is allowed to see, resulting in invisible viruses that are being hidden by the rootkit.  And naturally the rootkit won't let 
itself be detected either.  They're rarer but they are out there.  Mytob.AR and Mytob.AD use rootkits.
Anyway,
1) answer my question about what firewall you use.  Also, what Service Pack level does your Windows have?
2) run a LiveUpdate of your Norton 2004.  Go through all the configuration panels and max out the Heuristics, enable scanning within compressed files, and enable every other detection option too.
3) get today's 
Intelligent Updater and use that to top off your Norton definitions to the very latest version
4) get the manual 
McAfee scanning tool, and unzip it as the directions in my text file say.
5) 
Restart the system in Safe Mode and run the McAfee scanner and also your Norton scanner.  The McAfee scanner will vanish when it's done.  Write down precisely the names and variants of anything Norton reported.
6) Restart the system in normal mode and post what you found.  McAfee's findings will be in a file called 
C:\Report.html.
 Make 
sure you run your scans 
while the computer is in Safe Mode.