The plot thickened shortly after my first post, with Windows not starting in normal mode at all on the spurious excuse that my version of Windows is not genuine.
In the meantime, I went into Windows safe mode and deleted the files as per Captahab's post.
The computer's bizarre new attitude to the legitimacy of my copy on Vista led to the solution. I started by trying to 'activate' Windows by typing in the product key as requested on the blue screen. Cue Error Code 0X800700D. Hmmm. Unhelpful. I then opted for the online authorisation option. Cue next problem: Windows could not access the internet and did not know why because the Diagnostics Policy Service is not running. Hmmm. Fortunately the pc gave me the option of opening the wildly user friendly (
) 'Service Control Manager' which allows you to turn on 'services' you never knew existed. I manually turned on the ones that allow system diagnotics and Windows updates. Suddenly Windows woke up, and on re-boot things started working.
I allowed ESET to update the virus software and allowed Vista to update itself. An hour later and Service Pack 1 is installed and all is working again. I'll probably never know what caused the problem, but I am mightily relieved.
