Windows 7: problem: 3.2GB RAM usable out of 8GB!!

I Have Windows 7 Professional x64 with 8GB installed (2x4GB DDR3 Kingston). I installed the windows with the 8GB from the start. Windows recognizes 8GB installed but only 3.2GB usable.

In the Resource Monitor 4.9GB are "Hardware Reserved". I have a nVidia GTS450 graphic card, but the nvidia control Panel "System Information" shows that it uses 1GB from the card framebuffer and 1.4 GB shared system memory.

The M/B supports 8GB

I didn't find any memory remap option on the BIOS, but it clearly detects 8GB (the BIOS).

I tried checking (and unchecking) the "Maximum memory" option in the msconfig > Advanced Options. When i chek it it automatically appears "8192" (8GB?), but when i reboot and check again it appears "0" (which means auto??). Nothing happens at all.

Further PC specifications:

MSI H55M-E21 M/B (socket 1156)
Intel i5-760
nVidia GTS450
2x4GB DDR3 Kingston (PC3 - 10600 CL9 240 - Pin DIMM)

See the images below

Any Ideas?

P.S: please dont tell me to check if my OS is 64 bit. It is

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