how safe  from viruses is  the "limewire" web site for a family to
download mp3 files from ,   to a home PC which has an up to date
Norton Virus protection?

Not very. I used to work for a plumber, home office, his teenage son
had Kazza, Limewire and Norton, used to get viruses (?) all the time.
I kept telling the kid to stop, I'd uninstall the crap and get rid of
all the problems, but his father never understood enough to MAKE him
stop.

Finally the kid moved out, his Dad bought him a laptop for college, 6
months later the laptop was dead as a doornail. Of course Daddy bailed
him out, hired his Network guy at $100 an hour to fix it, geez, I
would have done it for $50 (LOL), he ended up spending about 1/2 the
cost of the laptop.

Keep that stuff OFF your computer. If you want music, download and use SoulSeek:
http://www.slsknet.org/
...and be sure to delete files within 24 hours...

Since we are using the word 'family'...would the better idea not be to
teach your 'family' that illegal p2p networks are not a good idea? 
Why not introduce your family to free, legal and virus free music at
Creative Commons licences sites or at places like
http://music.download.com/ which is part of the very reputable C|net
family of websites?

To answer your question more directly...You likely won't have a Google
Answers person answer you directly as they would be breaking the
policies of Google:

Keep in mind that Google Answers is not a substitute for professional
advice or services, nor will we knowingly provide answers to assist in
illegal activities. (Since helping you avoid virus' perpetuates
downloading via p2p which is starting to get more and more dicey:
http://www.pcpro.co.uk/news/81288/woman-loses-riaa-internet-theft-appeal.html

Scan EVERY file before it is ever played for the first time, do NOT
allow it to be previewed within Limewire or at anytime until you have
scanned it. (Personally I use Avast)  Scan with MULTIPLE scanners. 
Only download files that a: seem to have a file size that is
reasonable for the song (the latest Kayne West song should NOT be only
182KB in size!!!) you are looking for and b: have many many people
sharing it.

If you have found my answer helpful please indicate so before expiring
this question.  As I have said...due to Google's terms of service it
is unlikely that you would get a specific answer as you are asking
them to comment on an activity that is generally now being
prosecuted...

You should ask yourself how legal it is for you to use limewire to download mp3s.

Not safe at all.  First of all as previously stated, you are running
the risk of being caught by the RIAA.  Second of all, new viruses are
being created hourly if not faster.  The anti-virus programs cannot
protect against something that it does not know about.  So if a virus
was created today, downloaded today, you will be infected even if you
are up-to-date, as the virus protections are usually a bit behind. 
That is not the anti-virus company's fault, you can not protect
against something when you don't know what you are looking for.