Sometimes memory can be a little tricky to determine if it is actually defective. Since DDR memory is the most popular right now and it comes in DDR, DDR2, and recently DDR3. It also comes in different speeds and timing, which makes purchasing the correct memory for your motherboard cumbersome sometimes. I say this, but you might not have bad memory, but memory that is simply incompatible with your system. A great piece of software to help you determine this is Memtest86+. Since Memtest is a 32-bit application though, you should only test 2 gigabytes at a time.