Well, hardware allocates a certain amount of ram that it uses and that space is only for use by the hardware. Hence the reason only a certain amount is usable. Basically, you probably have an onboard video card that needs that 128mb of ram for it's video memory. If you really wanted to, you could reduce the amount but it can and will reduce all video performance of the card.
Here's a splice from microsofts website.
"On a computer that is running Windows 7, the usable memory (RAM) may be less than the installed memory.
For example, a 32-bit version of Windows 7 may report that there is only 3.5 GB of usable system memory on a computer that has 4 GB of memory installed.
Or, a 64-bit version of Windows 7 may report that there is only 7.1 GB of usable system memory on a computer that has 8 GB of memory installed.
Note The amount of usable memory in the examples are not exact amounts. Usable memory is a calculated amount of the total physical memory minus "hardware reserved" memory.
To view the installed memory and the usable memory in Windows 7, follow these steps:
, right-click Computer, and then click Properties.
View the Installed memory (RAM) value under System. For example, if it displays 4.00 GB (3.5 GB usable), this means that you have 3.5 GB of usable memory out of 4 GB of installed memory."
So if you want more memory available, you'll need to add some ram to the system itself.