Belated thanks for your replies. I thought I’d better stay off-line until this was fixed.
Turns out that the problem was;
Source: C

WINDOWS\Start Menu\Programs\StartUp\Reboot.exe
Description: The compressed file Reboot.exe within C

undo\backup.cab is infected with the Trojan Horse virus.
This means (I’m told) that, as the problem got into the Restore Files Norton couldn’t Delete it. I understand that. What I don’t understand is this. Norton warned me of an infection and therefore recognised the virus. If it was able to do that then why couldn’t it stop the damn thing in the first place. I’ve always got Norton Automatic Up-Date enabled. Even so, when I go to manual up-date there’s always something new to download.