I use an antivirus program called Antivir Persional Edition
, I found it to detect more viruses than Norton, it is also very fast, light on resources and free!
also, Norton and a lot of other antivirus progs are recognized by the OS and a lot of programs as an antivirus prgram, because it is from a big company, so a lot of viruses target it. Antivir doesn't do this, so it is not a target to viruses.
you said it comes up on logon/boot, so you might be able to find the .exe or .bat file that's causing this by going to start, run, regedit, then going to "HK Current User\Software\Microsoft\Windows\Currentversion\Run " see if there are any suspicious programs in the list, and delete at will. this directory is the list of all the progs that come up on boot, and displays the path. I find using the registry rather than Msconfig is better