EDIT: Before attempting anything, it's important that you have a working backup, too! If possible, ensure that you backup any important files (preferably to an external drive) before continuing.
I'm not sure if I've ever tried Ubuntu's USB installer option... I'm more familiar with the LiveCDs. If they are similar, though, does it give you the option to "Try Ubuntu" when you boot from USB? If it does, select it, then open a terminal...
EDIT AGAIN: Sorry, forgot to mention that some of the following terminal commands should be run with administrator rights.... I have changed the commands below to run as administrator.
Type in something like
and then mount the partition holding your / directory there:
sudo mkdir /mnt/Ubuntu
NOTE: Your partition may be named differently, depending on how your computer is set up.
sudo mount /dev/sda1 /mnt/Ubuntu
which should let you back into your Ubuntu install. Once in there, type , where username is the account that you want to set the password for. If you are unsure of what the exact username would be, something like should list your username(s).
sudo chroot /mnt/Ubuntu
And now, just type to cleanly exit the chroot, and then
. Finally, reboot the computer and boot from your hard disk.
sudo umount /mnt/Ubuntu
Hope this helps