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4GB RAM Installed 3.5GB Usable x64 bit Windows 7
If Windows says that less than the maximum amount of RAM installed is available (as it does in this scenario), the first place to look is in resmon, specifically on the memory tab. This will show you if the memory is "hardware reserved" (reserved by the BIOS for hardware devices) or not. After that, there are ways to determine who or what is reserving the memory, but that is usually an exercise in futility as you aren't likely to be able to configure the BIOS to get much (if any) of the hardware-reserved memory back.