Why doesn't my WindowsŠ PC use the whole memory[RAM] I installed?

The maximum memory[RAM-Random Access Memory] our system support depend upon the Operation System[32bit or 64bit] and Your Motherboard.

When you install 4GB ram on 32bit  Windows system your system will utilize only 3GB or 3.5GB .

There isn't a problem with the memory. Windows allows for 4GB of memory to be addressed, but this isn't 100 percent the same as having 4GB of physical memory. What happens is that some of the addressable memory (regardless of how much you have physically installed) is reserved for use by page files or by some of the devices that you are using, such as a graphics card, PCI card, integrated network connections, etc...so it's unavailable for use as normal main memory.

The maximum memory limitation varies by operating system; for instance, the 4GB memory limitation doesn't exist in 64-bit
versions of Windows [Which is used by High-End Users].
Memory maximums for current MicrosoftŠ Windows OS :

Windows 7 32bit and 64bit 



 

 

 

 

Windows XP 32bit and 64bit



 

 

 

Windows Vista 32bit and 64bit