2
make sure your cache/ folder is writable by the system (CHMOD 755 or 777, depending on your server setup).
If that doesn't work, remove the cache/adminmenu.php file, and go to the admin section of your site. It'll say that this is the first time you're logging into the admin section (this message is shown when the adminmenu.php file is generated), and that should fix that problem.
Herko