Try getting a Live CD of Ubuntu 8.04, and boot it up (may need to
change the boot order in your BIOS).

Then you can get root access by issuing the command:
ubuntu@ubuntu:~$ sudo -i
which gets you the prompt:
root@ubuntu:~#

From there, issue the following commands:
First, you will have to mount the installation hard drive, but first
you need to know the partition name to mount:
root@ubuntu:~# fdisk -l
This will tell you the Linux partition (not swap or boot) ID=83 of
your installed Linux (hopefully to the point where it was) and you
will need to substitute the /dev/xxxx below with the information
output from the fdisk -l command above.
root@ubuntu:~# mkdir /tmp/hd
root@ubuntu:~# mount -v -t ext3 /dev/xxxx /tmp/hd
root@ubuntu:~# cd /tmp/hd/var/lib
root@ubuntu:~# chown 106 gdm
root@ubuntu:~# chgrp 114 gdm
root@ubuntu:~# cd /
root@ubuntu:~# umount /dev/xxxx
root@ubuntu:~# exit

Then reboot the system.