Whenever I install, update or remove something, it fails and I get a message similar to what is below. After I restart, everything appears to work fine.

I just installed some updates using update manager, and here were the details shown in the "Package operation failed" window.

```
installArchives() failed: (Reading database ... 

[...]

run-parts: executing /etc/kernel/postinst.d/zz-update-grub 2.6.35-23-generic /boot/vmlinuz-2.6.35-23-generic
exec: 15: update-grub: not found
run-parts: /etc/kernel/postinst.d/zz-update-grub exited with return code 2
Failed to process /etc/kernel/postinst.d at /var/lib/dpkg/info/linux-image-2.6.35-23-generic.postinst line 1010.
dpkg: error processing linux-image-2.6.35-23-generic (--configure):
 subprocess installed post-installation script returned error exit status 2

```

See the [full log here](http://paste.ubuntu.com/542121/).

How can I stop this happening?