I have a computer with 4GB RAM. Why isn't it all usable? What does this mean?
Installed: 4GB, Usable: 2.9GB
It's because you have 32bit installed. 32 bit can only go so high. You need 64bit.
If you do have 64bit, 1Gb of RAM may be shared with the video card.
This is purely a limitation of the 32-bit operating system, and has been around since XP. Vista SP1 had an update that changed the dialog to show 4GB, but in reality, the OS was still only using 3.
The reason: Windows 32-bit can only use 4GB of memory. This applies to ALL hardware, including your video card.
If your video card has 1GB of VRAM, this means you're trying to use 5GB total. The BIOS often has settings to choose the amount of memory dedicated to your video card, and Windows will use as much RAM as it can in the remaining space.
Here is an external reference with people stating the same thing.