The BT Yahoo! browser was 'retired' 2½ years ago, with users being advised to switch to Internet Explorer, so I'm guessing that's what you're actually missing. Further, I'll guess that your browser is exactly where it's always been; you've simply lost the shortcut to it.
Go to Start > Programs and look for Internet Explorer. Right-click on it and select Send To > Desktop. You should then have the shortcut back again.
If you've got a taskbar at the foot of your screen, right-click on a blank space and then on Toolbars, to see what should be displayed. (Click on anything you want to see that hasn't already got a tick alongside it).
If you've not got a taskbar at the bottom of your screen, it might be minimised to zero height. Place your cursor at the very bottom of the screen. (It should switch to a double-headed arrow, showing that it's resting on the top of the taskbar). Click-and-drag it up to where the top of the taskbar should be, then right-click on a blank space on it and select 'Lock taskbar' to prevent it disappearing again.