Just a memory
One morning instead of booting up my wife's MacBook Pro was making an awful alarm noise. A quick google identified this a a memory problem. At iFixit I located the instructions to getting to the RAM modules. With access to the two RAM modules I could try the simplest fix, reseating the modules, which didn't help, and then trying each one separately. Voila, one works and the other doesn't so all we need is a new one. And how easy it was to get one, just click a button on the iFixit site and order it.
The part arrived days ahead of schedule via USPS's faster method. After popping the module in and replacing the cover etc. everything is working again. A quick check of 'About This Mac' shows we now have 4 GB instead of the 2 GB we'd been running on the last two days. Success!
This is a really easy fix. You need a really fine tipped phillips screwdriver to get the cover over the RAM off. I have a set I bought at Radio Shack. The RAM pop up when you spread the clips on each edge away from the modules. Then just pull them out. The modules have a notch in the band of gold contacts on the edge that goes into the socket, you need to match the notch to the socket. We left everything apart and ran with one module until the new one arrived.