Easiest method (will not work if GRUB menu password is set)

Another method is described at How to use a Ubuntu installation CD to gain root user access?

Boot-up computer

If GRUB menu is hidden, press Esc to enter the GRUB menu

If GRUB password is set, press p to unlock the GRUB menu

Select Ubuntu, kernel 2.6.12-8-386

Press e to edit the commands before booting

Select kernel /boot/vmlinuz-2.6.12-8-386 root=/dev/hda2 ro quiet splash

Press e to edit the selected command in the boot sequence

Add rw init=/bin/bash to the end of the arguments

grub edit> kernel /boot/vmlinuz-2.6.12-8-386 root=/dev/hda2 ro quiet splash rw init=/bin/bash

Press b to boot

Boot-up computer into Ubuntu Installation CD

At “boot:” prompt, add “rescue” to the argument

boot: rescue

Follow the instructions on screen

Read How to gain root user access without login?

To change root user password

# passwd root

To change main user password

# passwd system_main_username

grub

grub> md5crypt
Password: ****** (ubuntu)
Encrypted: $1$ZWnke0$1fzDBVjUcT1Mpdd4u/T961 (encrypted password)
grub> quit

sudo cp /boot/grub/menu.lst /boot/grub/menu.lst_backup
sudo gedit /boot/grub/menu.lst

Find this line

...
    password --md5 $1$gLhU0/$aW78kHK1QfV3P2b2znUoe/
...

Replace with the following line

password --md5 $1$ZWnke0$1fzDBVjUcT1Mpdd4u/T961 (encrypted password above)
                            

Save the edited file (sample/menu.lst_changegrubpasswordforgotten)

Read How to use a Ubuntu installation CD to gain root user access?

Assuming that /dev/hda is the location of /boot partition

# grub-install /dev/hda

If you need to reinstall GRUB, read How do I check disk space and view the partition table?

Launch System->Administration->Boot

Click on Add.

Choose a name such as "Windows XP" and the OS type is unknown.

Assuming that /dev/hda1 is the location of Windows partition, choose that from the drop-down list

Download Ext2fs