Different editions of Windows have limits in how much ram they will use. All 32bit editions of Windows 7 max out at 4GB of physical memory except Starter which maxes out at 2GB. Then some devices take a chunk of the the physical address space for their own use limiting it further, usually the graphics card takes the biggest chunk.
To use more installed RAM switch to the 64bit edition which for Windows 7 Professional has a 192GB physical memory limit.
Physical Memory Limits: Windows 7
The usable memory may be less than the installed memory on Windows 7-based computers