If you have a Trisquel live cd (or another GNU/Linux installation), boot into
that and open a terminal emulator. Then, with root privileges:
1) mount the installation for which you've forgotten the password
2) chroot into the installation you just mounted
3) use passwd to set a new password
I just double-checked that you can do this using my Trisquel live cd, and it
went like this:
trisquel@trisquel:~$ sudo su
root@trisquel:/home/trisquel# mkdir /mnt/sda1
root@trisquel:/home/trisquel# mount /dev/sda1 /mnt/sda1
root@trisquel:/home/trisquel# chroot /mnt/sda1
root@trisquel:/# passwd oldfolio
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
/dev/sda1 is my Trisquel installation
oldfolio is my administrative user
