Error:could not create the java virtual machine

I'm having an issue that is whenever I allocate more than 1G of ram, an error comes up:
ERROR:could not create the java virtual machine
ERROR: A fatal exception has occurred. Program will exit.
This has happened on my old windows 7 laptop and my new custom built desktop with windows 8. The laptop had 4G ram, My desktop has 8G of ram. So I should be able to allocate at least 2G ram. My oldest laptop had 8G ram, and it had no problem with the same code I put in to the launcher for 4G ram. My computer is only using 1.7G ram at the moment. I cant figure out how to fix it! please help
P.S: I also had this problem with all of the other versions after 1.6