Thank you both for your very prompt replies. Before posting my problem, I had already carried out the processes you both recommend. With system restore disabled, Norton removed W32.IRCBOT.Gen and the following files were quarantined. TFTP180, TFTP3132 and TFTP636.
Spybot S&D found three copies of Lsa but was able to remove only one copy (even in safe mode). I located the two remaining copies in the registry. In the two registry folders called Lsa there was an exe file called dynjngo. I then I posted my message asking for advice about deleting them from the registry. I therefore repeat my question:
Do I delete the Lsa folder or just the file?
My second question was: As Norton deleted several TFTP files, should I be concerned about an exe file called tftp which is in the Windows System32 folder? Kind regards