Why is usable RAM less than total RAM?

Why is usable RAM less than total RAM? "My girlfriend bought a laptop last week. It's a core 2 duo with 4 GB We installed vista 64bit, and one of the first things we did was right click on ""My computer"" to see gthe properties. Immediately we noticed something strange about her RAM, the line said: Installed memory (RAM): 4,00 GB (3,68 GB usable)"
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Guest [Entry]

"Two possible reasons:

the video card sharing RAM (using motherboard RAM instead of having its own, or using motherboard RAM to supplement its own)
the motherboard chipset does not support remapping (the PCI architecture traditionally ""owns"" a chunk of the top Gb of the bottom 4Gb of physical memory, the remapping moves this above where your RAM is actually sitting so the two areas don't overlap). This is common with chipsets that only support 4Gb of physical RAM in total."