sudo not working in konsole

The user permissions for sudo are not working. For example, when trying to edit or even read the grub bootloader file in Kubuntu 8.04 KDE4 as a normal user:

user@laptop:~$ cd /boot/grub
user@laptop:/boot/grub$ sudo vi menu.lst
[sudo] password for user:
user@laptop:/boot/grub$ cat menu.lst
cat: menu.lst: Permission denied
user@laptop:/boot/grub$

Even though I supply the root password nothing happens, i.e., vi is not invoked. However, if I logon to a command console as root, I can edit the files, etc. How do I correct the permissions?

Thanks in advance for any help.