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.