My dad has a windows 7 PC, and everytime I check the "Ask toolbar" or some other browser toolbar is there. I don't know which software update includes it, can I forbid application called "*toolbar" from installing, or at least to have windows ask for a password ?
remove the admin rights from your dad, turn it just a plain user.
then remove the toolbar, check the registry and filesystem to seen if it really gone. check this for help removing it:
if it show up again, its something you already have installed (probably some malware that is cashing for each toolbar installed).
If its installed just on your dad profile, its something he is doing/visiting that installs that (probably via a security problem in a browser, java, acrobat reader, flash, etc, etc)
if doesn't show up again... problem solved :)