Ok I've re-installed avast completely. When removing old version I found files relating to avast verion 4 so got rid of them as well.
I have installed new copy and registered it but still won't update. Slightly different though, instead of vista error message appearing, it seems to update by saying "saving package" but then stops abruptly. Message within the program itself on the update page then comes up "Last encountered error. Invalid file signature. Setup will terminate."
i went back to Programs & Features via Control Panel and tried repairing it - same error message as last time. Then tried update option at same time - again seemed to work ok but then at the end it came up with "Update finished. There was an error while changing product configuration".
The program is now at virus definitions 101011-0. I am worried to use internet now with definitions that are so out of date.
Any advice would be great