Edit (sorry, I couldn't see a delete button) - problem solved.

Only Administrators and Moderators can delete posts et al.

It's a shame you removed the problem description and the remedy. It may have helped others.

It was a mistake to be honest - I had only meant to delete my Hijack This log and post a message saying the problem was solved, but by then my nerves were frazzled and the cursor got carried away. I apologise.

It turned out not to be the Windows Installer in the end that was causing the problem, but the hidden remnants of the long deleted Sony SonicStage program suddenly coming to life and deciding they wanted to play again. You just cannot get rid of that blasted program, in the end I had to reinstall it to solve the problem (which was the Windows Installer on a constant loop of pop up messages trying to install a component that didn't exist , then failing, then trying again. Non stop)

If I have any advice after this experience it would be to not install SonicStage in the first place!

Thanks for the feedback.