Fix Usable Memory is less than the Installed Memory – Windows 7
Some users may have problems when it comes to the amount of ram installed on their computer. Example: you have 8 GB of ram installed but Windows 7 is using only 5 GB Usable Memory, that problem is very common.
1. Press the Windows + R keys to open run command and type msconfig and press enter.
2. Select Advanced options under Boot tab.
3. Check mark Maximum Memmory option and type the amount you want to use and press Ok and restart your pc to see the changes.
This is expected behavior on computers that are running Windows 7.