You may have installed a program that sets search/toolbar to Yahoo. Bit of a pest, that.
Have a look in browser for View tab, if you see Yahoo, uncheck it, if you see Google, check it. Close browser.
My advice is also to check Control Panel/AddRemove, and look down bottom of list for Yahoo toolbar. If there remove it, it is resource-hogging.
Open browser again and go to google.co.uk.
Open Tools tab/Internet Options ... In Home page select "current" or whatever you like and apply/ok.
If you want Google toolbar, you may have to install it. In Tools/Internet Options/General/Search settings you can check what is installed as browser addon (this is for Internet Explorer 8 latest version). My advice is to check Yahoo is no longer there.
Make sure Yahoo toolbar is not selected when you install other programmes!
Your cookies probably expired or deleted for ASB. You have to login again to save cookie. If you have to log in every time, you may need further help setting browser cookies!