Finally got a chance to try this tonight. A neighbour popped up on Facebook chat asking for help because her laptop was claiming to be infected, and she couldn't get on the internet any more.
I told her to bring it over, sure enough it was infected, with a rogue anti-virus, I think it was called Windows 7 Anti-Virus or something like that.
It wouldn't let me run MBAM, so rather than mess around trying things, I downloaded Microsoft Standalone System Sweeper Beta , booted from it and it found 3 severe/high alert threats that it dealt with.
Once it restarted I installed MBAM, SAS and avast using a cd that I burnt in May, so it has old definitions on it, but they still found a lot things to delete.
Tomorrow I will connect it to the internet, update the definitions and then scan again.
The main thing is though, Microsoft Standalone System Sweeper Beta got me to a stage that the computer was more usable.