Most PC users are familiar with the 4GB system memory limitation of 32-bit Windows Operating Systems, which is why 64-bit computing has become the standard for computer enthusiasts. Occasionally though, a computer system will not report the correct amount of RAM installed. In this article, our 64-bit Windows 7 test system had 6.00 GB of installed memory but indicates only 4.00 GB usable RAM available. While some may dismiss this as a case of defective RAM, you might be surprised by the culprit. Benchmark Reviews troubleshoots Installed Memory vs RAM Usable by Windows. Diagnosing system memory problems is usually a strait-forward chore: test each module individually using free tools such as Microsoft Memory Diagnostic or Memtest86+. This process usually roots out the defective module, and allows you to move on with replacement. Unfortunately, not all problems are so easily solved. In some cases, the computer system will work without errors or crashing, but the amount of installed memory is more than what Windows reports as usable. This is where troubleshooting can take you to task.