I was using my MacBook last night and all of a sudden everything started to run very slowly. I powered it off and powered it back on, the Apple came up and pinwheel was spinning indefinitely. I've tried powering on and off, resetting PRAM and PMU and that didn't work. I finally found my OS X disks this morning and tried to do a repair via Disk Utility, but it reports an error, "invalid b-tree node size" and won't repair the disk. I can't reinstall OS X, my HDD doesn't show up. I don't have any files backed up (yeah, yeah ... I don't have a way to do this right now). Is there any way to fix this, or am I screwed?