Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
That is the self-defence mode possible, but the program should uninstall from windows add remove programs when in Normal boot mode. Are you getting any errors when you try to uninstall in normal mode ?
Are you uninstalling and installing from an account with Administrator privileges ?
I don't use Vista so I don't know if the UAC would get in on the act when you try to install, perhaps you could try right clicking on the avast setup file and select run as administrator