It used to be that the only way was to get rid of all traces of Windows desktop search via add/remove programs.
That was a year or so ago....... Maybe there is a better way now.
I don't know if it is possible to remove the bar from the screen, but you can completely disable the search function (from taking system resources and indexing, etc) by going to My Computer (right click)>Manage>Services and Applications>Services, then click on either "Windows Search" or "Windows Desktop Search" and change the startup type to "Disabled" and set the status to "Stop". You will then never have it run again.
You would have to manually edit the registry to get the search bar at top to disappear, I think.