4GB RAM installed but only 3GB usable?

Hello,

I have 4GB of RAM on my computer, but system info (right-click "Computer" -> "Properties") states:

Installed memory (RAM): 4.00 GB (3.00 GB usable)

Any ideas where the extra 1GB is hiding?

I used to have 3GB installed, purchased and extra 1GB and ended up back at square one (by the looks of things)...feel a little cheated to be honest!

Thanks in advance,

Al

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For reference, my system setup is as follows:

Windows 7 Ultimate
Dell E520
2.8ghz Intel Dual Core
4GB RAM
GeForce Palit 210

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