1st thing u need to do is uninstall yahoo toolbar from control panel. Then see if any remnants are left in c program files folder. then go to C:\Program Files (x86)\Yahoo!\Shared and rename the npYState.dll to anything like 2323 and remove its extension name also. So now both yahoo toolbar and its plugin are disabled/removed.
then try all steps mentioned here - http://forums.anvisoft.com/viewtopic-45-1165-4360.html
if still not solved then do a simple system restore in "safe mode" to the point before u installed the yahoo software.
and also u said its coming again and again so that might be related to startup or some services of it still present.
try installing CCleaner and then go to tools > startup and in there remove all yahoo entries if there from the windows, internet explorer, scheduled tasks section. And open services.msc (windows + r then type services.msc and enter). and scan there for any service named or related to yahoo and disable that and
delete that service
and also to manually set google as homepage without going to internet options
try this link - http://www.google.com/intl/en/services/hp/index.html
Google > Yahoo (i.e google greater than yahoo) and more powerful so it should replace yahoo and set itself as new home page.
And in future if u want to install yahoo again then please always uncheck -make yahoo my default search engine,homepage and install yahoo toolbar.
Same for other softwares too. Always don't go on clicking next next next and install.
Try all the things I said & please don't skip any of them & always have a good approach that I will fix it and not like o this tutorial looks so long. wonder if all the tips mentioned are gonna work. lets try the simple ones.
Never ever do that. (Just suggesting atlast v can give all the suggestions but u would have to do and try them perfectly as mentioned)
Hope this helps. Good Luck mate