i haven’t had that problem and don’t want to try to reproduce it. what i would do in your case is booting the machine from a liveCD, mount the installed file system and see what grub specifies as root file system. there’s something wrong, apparently.
i suspect the liveCD with KDE 4.5.1 has the new kernel update, and something went wrong during kernel or grub installation. to me it looks as if grub gets it wrong specifying the root file system. should be easy to fix once you get in there with another linux instance.
It’s possible that this file is incorrect and is pointing to the wrong partition. A clue would be perhaps available if there is a menu.lst.old also, which you should compare. If you post the contents here we can check.
Also supply the terminal result of: