I have 4 GB of RAM installed, but why are only 2 GB are usable?

"Use MSConfig → Boot → Advanced.

Is the box 'Maximum Memory' checked?"

"Ah, I see you have a Dell. Mine is similar -- a 4GB Dimension 5150 system that only reports 3.3GB as installed, all of it usable. This was the case under Vista-32, Windows7-64, and assorted Linux distributions of both bitness-es.

In my case, this is a limit of the chipset -- the onboard video appears to sit on a large part of mappable memory, making it unusable to the OS."