Perhaps if you could tell us what method / software you are using to create the virtual machine(s) (virtual box, VMware Player or Workstation, XPmode, etc.,) we could be more helpful most have a method or means of configuring the memory allocated to a particular instance of a VM, keeping in mind of course the the software itself while running would likely consume some additional memory. What is telling you that you are using 50% of presumably the host machine's available memory?