you could check this out : Maximum usable memory for 32-bit Win 7
As stated there, your problem is the 32-bit OS, that has a restriction on how much memory it can map.
I made a clean install of 64-bit Windows 7 to my comp, and now all the RAM is in proper use. Before doing the clean install, it's useful to have an external drive where you can transfer most of your files with "Easy Transfer". And you'll need all the installation files for your applications, because they'll get sweeped in the process.