Now....!
Thanks again essexboy, this help is greatly appreciated.
Here's an update:
After following your instructions, I rebooted (see my note on that point below...). Up until now, I could see the siszyd32.exe file in both CCleaner and in the Startup list via Windows Defender, and I could disable it, but not delete it. This time, I could still see it, but was able to delete it easily. Whether I have just deleted it from the startup list and its still there somewhere or its completely dead I'm not sure. I have attached the Combofix.txt and log.txt files for your info.
Anyway, CPU usage seems to have significantly improved (between 5% and 20% when I'm using the same programs where previously it was generally between 50% and 100%).
On the reboot, just one thing to note - the previous 2 times I ran ComboFix, it didn't ask / suggest / require a reboot, but this third time I couldn't open either Firefox or IE after the process had finished (a message popped up saying I had selected a registry item that was marked for deletion). A reboot seems to have gotten over that, but anyway, there you go... just for future reference in case its of interest.
MP