After contacting Reallusion Support,they asked for DXdiag in text and file format.Two days later they said that as far as they could tell, there was nothing wrong with my system or the iClone software.They also said they had not come across this problem before.
Trying to get to the root of the problem,I decided to start from scratch.
I will pass on what I did,because for the last two weeks,the problem has never occurred again.
I'm not sure if this will resolve everyone else's problems.But so far it's working for me.
First,I uninstalled iClone.
I may have had some conflicting software,so I removed and uninstalled old programmes, or programmes that I did not use,or were downloaded from "Free Sites".
I defragmented hard drive,system clean up and system restore.
I also uninstalled AVG Anti-Virus and now use only Windows Security Essentials and Firewall.
I then reinstalled iClone.
Repeating what I said earlier,so far (two weeks) no problems,no shutting down suddenly,no blue screen,no freezing or slowing down.
I had tried everything else, but this simple solution has resolved my issue.