For example you have a 4GB of RAM but it only shows that only 3.87GB can be used.
I have a solution for that.
First if you are a Windows 7 user, Click Start button then type msconfig
and if you are a Windows 8 or 8.1 user just simply type msconfig
After that click the Boot tab
Then click Advanced options
Check Maximum memory and enter your total RAM.
If you have 1GB of RAM, enter 1024. If you have 4GB of RAM, just multiply 1024 by 4 so on and so forth.
I hope that I solved your problem 🙂