Good enough. Thank you again. And thank you for your candor.
Sorry to add a contribution… The virus W97M.thus.gen (named according to ClamXAV), a.k.a. W97M.Thus.EQ.gen (according to Norton), has been detected on a USB key, first on a Mac.
However, Avast still does not detect it (although Norton on another PC actually does). I am using Avast 4.6 - antivirus base 0616-0 (17 April, 2006) on a Windows 98SE PC. The scan is set on “standard” and there is no exclusion.
I wonder ??? why Avast does not see the virus while others do. Any advice, explanation, even comforting joke would be welcome. 
Thanks in advance.
The virus name perhaps provides a clue W97M.thus.gen and W97M.Thus.EQ.gen the gen stands for Generic so it could well be a new variant that is caught by generic detection.
You don’t state what the file name or location that is infected ?
That could perhaps avoid a Standard scan if it was in an archive file and you hadn’t chosen to scan archives ?
Because the Standard scan doesn’t scan all files but those most likely to be infected, etc. it may not have scanned the file, a ‘Through’ scan would scan more of your files and would obviously take much longer.
Did you try a right click context many scan with ashQuick on the suspect file in question ?
If after testing with ashQuick.exe it still isn’t detected they send it to avast to be included in the VPS.
If you are not getting a virus warning that you believe is a new or undetected virus, then if you can zip and password protect (‘virus’, will do) the suspect file and send it to virus @ avast.com (no spaces), or send from the chest.
Give a brief outline of the problem (possibly a link to this thread), the fact that you believe it to be a either a new or undetected virus and include the password in the body of the email. Some info on the avast version and VPS number (see about avast {right click avast icon}) will also help.
Thanks for the answer.
Unfortunately, the maybe infected file is a Word document, directly on the USB key (and later on my desktop! Gosh! :-[), with no shell such as a Zip or RAR or else.
I tried to scan it manually (right rick on it and use of the contextual menu to scan the file - the program name you refer to does not evoke anything to me) and Avast didn’t find anything. Hence my question.
I’ll do exactly what you suggest tonight. If, as I suspect, no virus can be seen, I’ll send all the information as requested.
Thanks again.