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:
How do I remove the ask.com toolbar?
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 :)
2"I don't know which software update " very likely java. – Journeyman Geek – 2013-02-22T11:30:09.320
which is weird, because I first heard about it when a site using java did not work as long as that toolbar was installed – Manu – 2013-02-22T11:32:46.410
This is a brilliant idea, although I don't know how to do it. – pjc50 – 2013-02-22T11:45:10.633
This can't be done. Would be easier to just not allow your father to install applications himself. – Ramhound – 2013-02-22T12:47:12.773