I'm running Xubuntu 12.10, and I recently suffered a crash. When I tried to reboot, the system froze at "Stopping System v Runlevel compatibility" The advice for this tells me to go to tty1 via ctrl-alt-F1, but when I do this I can't login. I enter my username and password, and the system says that these are wrong. All of the suggestions I can find for getting around this assume that I have some kind of access to the system. I've got it running via a live-USB of 12.04 at the moment, but I don't know how to alter anything from here.
Try to reset your password, here's a guideline
If this doesn't help, it may be fastest to install a fresh Xubuntu 12.10 (even though Linux can always be fixed ;-) Be sure to back up all your data, including hidden folders (starting with .) in your home dir that contain your settings. To my experience, restoring these settings by copying them back is safe as long as you stay with the same software versions (i.e. those that come with Xubuntu 12.10 in your case). Also back up your .deb packages in /var/cache/apt/archives; use them to remember what software was installed. (Tools such as gdebi or APTonCD may help.)
For backing up, consider copying the entire partition using gparted from the Live CD. Instead of backing up, consider replacing your hard disk by a new one, and then connect your old disk externally using some suitable USB - SATA adapter in order to copy your data back (fastest)