The computer did have Norton Anti Virus on it, but this was removed with the Norton Removal Tool prior to installing Avast 4 Home. I put Avast on to my Sons computer a few days before and again removed Norton Anti Virus and there have been no problems.
If for some reason this had been caused by a bad uninstall, why would it effect the clean Maxtor drive that I tried to carry out a new install of XP on.