Firefox hijacked by yahoo

Firefox has been hijacked by
http://au.yhs.search.yahoo.com/avg/...oo_avg_hs2-t-web_au&p=au.yhs.search.yahoo.com
It causes disablement - maybe because there is a conflict between it and my chosen home page, which is STILL showing as the one I chose. Sometimes my chosen home page wins out for a short while, but then gets replaced by the yahoo search; sometimes my own does not even get a chance. This conflict appears to disable other browsers from accessing the internet as well. I have received notifications like "A program on your computer has suggested a new default search provider for Internet Explorer." and even a dialog box with a choice of whether I really want to change. It is to no avail, YHS remains dominant. I've even replaced 'default search provider' in the Registry with my own choice, but after rebooting Yahoo is back in that key. How can I get rid of YHS and get my own choice back ?