Why are only 3gb of my RAM usable if I have 4gb installed?

A 32 bit operating system can only address 3 GB of memory. You would need a 64 bit processor and operating system to address 4 GB of memory.

32-bit and 64-bit Windows: frequently asked questions - http://windows.microsoft.com/...=windows-7

!GB of RAM is used by Windows and others RAM-consuming programs.

To upgrade to 64 bit means a complete wipe and reinstall no short cut for that

One of your sticks might be damaged

32 bit windows can only address about 3.25GB, and in many cases, on board video uses 256KB, leaving ony 3GB left for the system.
To make use of 4GB or more, you need 64 bit Windows. Even then, 64 bit Windows with 4GB ram will only show about 3.75GB usable - the rest is for the system or video.

Because some of it is already being used by the operating sytem to even run