I need your help diagnosing this early 2007 Macbook Pro running the latest version of Snow Leopard. It isn't mine, rather my sister's, so I don't know the full details of everything as I don't see her all the time. Last December, I swapped in my 160gb 5400RPM SATA drive from my late 2006 MBP into her MBP as I had just bought and installed a 500gb 7200RPM drive into my MBP. I installed a fresh copy of Snow Leopard onto her machine, patched it up, and sent her on her way. A few weeks ago, she came to me saying she was having trouble booting into OS X and was just having trouble using her laptop in general. The hard drive wasn't making any erratic noises, so I figured the system install had corrupted itself. The problem was I wasn't able to boot to the Snow Leopard DVD. The DVD would show up at boot when holding down the Option key to show boot devices, but when clicked, it would show the Apple logo, then the "prohibitory" symbol and the fans would spin up on full blast, with no further progress. Booting into Single User mode, I was able to run disk repair and after running it over a few times, I was told the drive was without errors, the laptop booted fine (minus the booting to Snow Leopard DVD) and I gave the MBP back to her. Fast forward to today, she's having issues booting in to OS X again. It'll show the Apple boot logo, but the small spinning circle will freeze, and the whole laptop will come to a halt. I still can't boot to the DVD either, with the same symptoms as above. I'm sure I can boot into Single User mode, run Disk Repair, and probably be good, but I want to attack this issue at the source. Is the hard drive I've got in her MBP going bad (I can't hear any of the erratic noises you hear when you know a drive is going bad)? What else could it be? What about the inability to boot to the Snow Leopard DVD? What is the problem and how can I fix it? Thanks, and sorry for the long winded post.