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edit-> ^^^^^time warp. love those simul posts.... 30 seconds to write a post and find a new one with the same answers....
adminmenu.php isn't a file at all in the core install. It gets generated all the time in the /cache directory.
Best advice is to empty the /cache & /templates_c (except for index.html) directories. This will start the whole caching process clean and should generate a new adminmenu.php file.