I had the file folder with the question mark keep appearing, and a reinstallation of the operating system didn't help. Starting up with the Installation CD, I saw that the hard drive appeared to be missing. I knew it was fine, though (it was checked on a USB adapter), so I suspected it was the cable.
I'm not a computer person, but the repair was very easy! (The most I had ever done before was replace the battery) It took less than 20 minutes to replace the cable. And after I restarted the computer, it was up and running smoothly right away!
The iFixit guide is very helpful! Definitely check if its the hard drive or the cable that is the problem before you buy replacements.