Thanks to all for the replies. I've gone through the horribly time-consuming procedure the Geeks Forum recommends, and although AVG no longer finds a virus on my system, if I do a Systems Files scan, the nasty virus box appears along with a message about a change to the MBR.
Can anyone explain whether this is (as I suspect it is) a problem?
I'd really like an hour or two alone with the *** cretins who create these damn viruses, preferably with a can of petrol, a vise, a blunt, rusty razor blade and a match.