Just yesterday my 2008 MBP gave me the prohibitory sign upon restarting. I have been using some external USB HDDs lately but haven't deleted any system files or whatnot. To make a long story short, I've tried resetting the PRAM, booting in safe mode (where I'm writing this from) and using Disk Utility to repair permissions and check the disk. I also verified the HD and it said it was fine. I have NOT yet tried re-installing from my Snow Leopard DVD. If I do this, I just want to make sure it won't delete anything because I haven't used Time Machine in a while so I really can't afford to lose any data. What system files could possibly be fixed by re-installing the OS? Lastly, what do I do if this step doesn't work? I don't think my HDD is going bad. I haven't ever dropped it or abused it in any way over its life. Thanks much for any info/advice!