The virus in question was - win32:Trojano-281[trj] and I don't believe it is a new virus.
I only found out when I was copying my Firefox profile to a backup location
and Antivir (which I had only installed a few minutes before) popped up and told me there were signs of the above virus in my Firefox cache.
I still don't understand how it got there or why avast! didn't detect it.
I only re-installed XP the other night and so far I have been unable to to update avast! because I keep getting disconnected every time I try (read my previous posts in this and other threads.
I so want to solve these problems with avast but after a month of these problems on XP upgraded from win 98 and again on a fresh XP install I've just had enough and am now trying out Antivir (even though it takes about half an hour to download an update).
As I have said many, many times, since switching to XP avast! is the only thing that has caused me any trouble.