Quote from Brosh J.:
What hidden files?? Please explain.
This only applies if you have previously installed a version of Opera. If not, you should get a clean install without the banner.
(I don't know why you would, but if you've never installed Opera and got the ad banner anyway, I'd follow the procedure below.)
When you "Delete Programs" out of Control Panel, it doesn't every file in the program. A few are saved, like for settings, favorites/bookmarks, etc. You have to delete these first to get a clean install.
Look under Help in your OS and find out how to "display hidden files"... then Search Files & Folders/ "opera*.*", and delete all of them your search displays.
Then, go back and uncheck what you checked to "display hidden files"... this is a system safety issue so you don't accidentally delete important files.
Then, install Opera and you're in business.