Can you think of anything that you've installed recently that might be the culprit? Also, after the failed installs, I presume you're manually deleting the folder that might be left in Program Files? Also, you may need to manually edit the registry to remove any relevant entries.
Sorry I can't be super helpful, but these things always end up being a real "detective hunt" trying to run down various symptoms or possible causes.
If you can get your hands on a spare HDD - even on loan - I'd be inclined to try an install onto that. However, it still sounds software-related, and I wouldn't rule Norton out. Although it may sound like blasphemy, you could uninstall Norton completely & try installing with no AV to see if that works. You'll need to do a restart & download the Norton uninstaller tool & run that after the reboot, as Norton leaves all sorts of poo behind & tweaked settings.
See how you go & keep us posted.
EDIT: Had to laugh at the forum system that "bleeped" my use of the word c r a p and put in "darnation"!

I changed it to "poo" instead...
