If it's an x86 machine I recommend trying the Ultimate Boot CD (assuming of course it can still boot to CD and that you have a way to burn it)
http://ubcd.sourceforge.net/
It has lots of tools that might help. I've used it to fix a hosed W2K install, meaning that I was able to get it to run long enough to copy off the important data before wiping the disk and "freshening" the OS.
For a Mac, boot an install CD (OS X preferred) and run "Disk Utility" from the CD. From there you should be able to verify or repair the disk or if you have a backup image, you can also restore the image as well.
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swiftly flying