file format problems

[Barry Tepperman]

I have become completely dependant on Ubuntu for all my personal and most of by professional computing. Have successfully upgraded(except for PCMCIA wireless cards - another story for another time) from 5.04 - 5.10 - 6.06. After the most recent software update (which included a kernel upgrade to 2.6.15-26-386) now find myself unable to reboot. The error message IMMEDIATELY AFTER attempting to load initrd in the startup sequence is "Error 16 : Inconsistent filesystem structure; press any key to continue....". The "press any key" gets me into the GRUB menu of current and previous kernels used with and without recovery mode - but none of them get me past the same point. I would - at the very least - like to salvage my data, and preferably get the system restored and running. 
 
So.... how to do that? I think running fsck would probably do it, but if I go to comand line at the point of this error fsck is not found. Do I need to boot from a UbuntuLIVE CD as a recovery disk? Do I just write the whole thing off to experience - OBVIOUSLY not the preferred approach?
 
Grateful for any suggestions. Perhaps once this is resolved I'll ask about the wireless card - but right now that's trivial.
 
Barry Tepperman