Hi,
Recently I tried to update my version of Avast to the latest version but had no luck. I'm sure it's an issue with my particular machine and software / configuration, but may I make a suggestion?
I was able to re-install my entire system exactly as it was before it crashed with all my programs, files and settings (not just to factory vanilla) by doing a image backup of my Windows Vista partition.
I use CloneZilla because it's free and it has saved me countless hours of frustration because if my system breaks for any reason I simply recover it from my last image backup (which I do at least weekly).
I always keep ONE tested (successfully recovered with it) image and then about 4 progressive recovery images. In case of system failure, I start with the latest and if necessary work my way back. Clonezilla has not failed to restore for me yet on the first try....so I stick with it.
There are other tools to do this such as Norton, Acronis and others.
But you will be glad you did an image backup if your system breaks and System Restore doesn't help.
Hope this helps someone.