as Gaz said, or you are running to many app's at the same time. Virtual Memory is disk space that is used for temporary storage of information if you do not have enough RAM, but it is slower than RAM - so the more you use it the slower the PC can run. A RAM upgrade is always a good route to better performance - depends how much you have at the moment.
Best bet is to set VM to system managed -
follow this link - but make sure it is set to windows managed, rather than custom as this guide suggests, this keeps it simple.