Thank you both for taking the interest to respond.
Eddy: The reason for multi-boot system is so that I can work on different systems that we have - I can assure you it isn't for pleasure!
Michael: thanks for showing me where to find the info on FRST etc - I couldn't find it myself.
In the end, I found that instead of warm restarts, a cold restart + F8 gave me options, so going back to last good config, and subsequent uninstall of Avast sorted it for me. Avast wouldn't uninstall normally so had to use the removal tool to get rid. The key was a cold restart which I should have thought about earlier.
But thank you both, and regards