Thanks for the tips. In the event the damage was more serious than I at first thought, or it was progressive, since it got to the stage where I had to format the old hard disc (again). I think the virus actually renamed the C drive with some odd characters so nothing could access it.
It was a bit strange that for about a year I had run with no antivirus protection with no discernable ill effects but within 3 days of installing this stuff - ostensibly up-to-date - it was totally f**ck*d. Hmmm.
Oh, to know where the bastard lives that wrote it!