OK, it's taken me a while, but I have run the AVG uninstaller, twice, searched for and removed everything with avg in the filename, run ccleaner, run ccleaner to fix the registry, uninstalled avast and re-installed it.
Still the same! runs ok for a bit then suddenly goes up to 98% cpu usage and everything else stops.
Any other ideas. Doesn't seem to happen on the desktop I loaded it to about the same time.