This happens, IIRC, because something causes Windows to get confused and thus essentially thinks that a major chunk is reserved by other hardware! I heard that bad contact with RAM-related hardware can cause Vista and later to get confused like this. (That's despite you should get errors or a boot failure when that type of hardware issue is present.)
Is your system newly built and has windows 8 as its first os? Well then i suspect it is not os related unless you try installing windows 7 x64 to try it out. Try reseating the rams and see if it works. Others also tried reseating the cpu to its socket and it reportedly solved the problem. If it still doesn't work, your ram might not be compatible for your motherboard. Others had their ram sticks replaced with other model and had their problem solved.
ummmmmm It seems to be some times graphics cards and other devices affect memory limits, It also depends on your motherboard ,how much it support the RAM,also other member saying the BIOS setting and some times integrated Graphics processor. The amount of memory supported by Windows in all its different versions not only depends on the operating system. There are some things to consider. For example: you should check with the manufacturer of your computer which is the maximum amount of memory that can support the motherboard and processor in your computer. For Example: I have an Acer Aspire 7750z with a Intel core B940 Processor and came with 4 GB of memory ram divided in 2 slots 2GB each. According to Acer Computer this system supports up to 8GB DDR3 1066 Memory Ram that's what they say, but According to Intel.com This Processor with integrated Graphics will support Up to 16GB DDR3 1333 Divided in 2 slots 8GB Per each slot Memory. So you must check first how much memory that can be supported by the processor if you are using 2nd generation Intel Processors you will be surprise then you will be able to know which operating system is the perfect one to support your hardware and take advantage of the power of memory ram. Right now I'm using Windows 7 Professional with those 16GB of Ram 15.9GB (Is actually usable) because some memory is used for the Integrated Graphics processor. In some cases your more than likely running a x86(32 bit) application on a x64 machine which would explain why that 32bit app only recognized 4 gigs of ram and not more. For an application to use more than 4 gigs of ram it will also have to be x64 bit. How graphics cards and other devices affect memory limits Devices have to map their memory below 4 GB for compatibility with non-PAE-aware Windows releases. Therefore, if the system has 4GB of RAM, some of it is either disabled or is remapped above 4GB by the BIOS. If the memory is remapped, X64 Windows can use this memory. X86 client versions of Windows don’t support physical memory above the 4GB mark, so they can’t access these remapped regions. Any X64 Windows or X86 Server release can. X86 client versions with PAE enabled do have a usable 37-bit (128 GB) physical address space. The limit that these versions impose is the highest permitted physical RAM address, not the size of the IO space. That means PAE-aware drivers can actually use physical space above 4 GB if they want. For example, drivers could map the "lost" memory regions located above 4 GB and expose this memory as a RAM disk. Code: In Run command type msconfig than see this option to selecting boot tab, as in screenshot :
To all, Thanks to all your help, buddy. This desktop has NO problem with memory when I installed Windows 7 and it got in full memory 16 GB. This problem happened as it's been installed Windows 8 x64 OS.