Okay, and still no luck with yr install of avast 2bad?
If the Norman uninstaller will run, regardless of what it says, then run it anyway if you can.
With avast 5, the uninstall utility for v.4.8 is bundled into the install process - you are prompted to uninstall and then Restart - after which you continue with the install of the new release. I think better this way.
However, you can uninstall v.5 independently using the avast 5 uninstall utility and you should do this.
Then continue with yr install.
I think now you should download yr install package again, making sure you get a good uninterrupted download - (or if confident with the download you already got, stick with that).
First, disconnect from the internet, and install yr avast while offline - you can always update later.
Turn off any firewall or any additional program that has potential to block - as that program may be carrying a record of yr previous configuration. And try install again.