Windows 7 using 24GB memory

Credit: bilbat
i3/i5/i7 'Missing' Memory
If you have 'X' amount of memory installed, and either the BIOS, or Windoze is only showing 'Y' amount available, the very first thing to do is: pull your CPU, get a flashlight & magnifying glass, and check your socket for bent or distorted pins:
Click to zoom
Nineteen times out of twenty, you will find 'em! Excellent pictorial guide to installation/examination from Intel.
...if the pins are not too badly mangled, they can often be 'teased' back into place, using:
a jeweler's tweezer (most large hobby shops)
a jeweler's screwdriver (any dollar store)
a dental pick/tool (often found at dollar stores or hardware store counters)
or
a 'pin-vise' with a sewing needle mounted in it (a 'pin-vise' is a little thing the size and shape of a kid's crayon, that has a tiny chuck at one end, much like a drill chuck - they are used to drill ultra-fine holes in small work, and can be found, again, at any large hobby-shop or hardware store...)
...just takes a steady hand, and a bunch of patience