The problem reported a few days back recurred in a different way recently. At random intervals, the laptop would freeze as the CPU was taken over by an unknown process. Using ctrl-alt-delete to open the task manager was no help at all. Various tasks such as iexplorer were flagged at 98-100% of CPU but stopping them (eventually) did not solve the problem.
Too many times, I would have to kill the machine dead by holding down the power button.
I rather feared that the spectre of a full XP-reinstall would be forced upon me. But I decided instead that the problem was with the Microsoft Indexing Service, via cidaemon.exe. This was, after all, exactly where I had had the problem before.
I therefore turned the service off using the control panel (administrative tools - services - indexing service => disabled). I use Google Desktop to search for local files anyhow.
Postscript: no sooner was I celebrating 24 hours of the problem apparently being cleared when it recurred! I have noticed, though, that whatever process is grabbing 100% of the CPU then releases it after a few minutes. So I walk away from the problem, get a green tea, come back and restart the machine. Seems to work provided life is not too urgent!