i see that im now required to use a password to open avast . is this as a result of updating to 542 or, as a result of updating to 542 on my wife`s user id . Cant see anywhere to turn it off? 
I take it that you installed Avast under you own account with admin rights (windows xp home) and that your wife has a restricted account(?), if so you must use your account when updating.
On XP pro, always install and upgrade Avast while logged in as admnistrator
Maybe your wife set a password…
If I remember correctly, some time ago an user had the same trouble and solved it uninstalling / boot / installing again / boot (like Eddy said, all did with the administrator account) 
thanks guys . wont do that again :o