Virtual memory is used when you are running low on RAM memory. You computer will then use the hard drive for temporary memory once your regular RAM is getting used up. Hard drive memory is much slower than RAM and your computer allocates a certain amount of hard drive space to virtual memory in what is called a pagefile. If your hard drive is also getting full then that's less room for virtual memory usage. Usually a reboot will flush your memory and take care of it but if you are getting this message all the time you may want to invest in adding more RAM to your computer.
You can also increase your virtual memory size by adding a user defined pagefile higher than the set system amount. This is done by going to your System properties in Control panel. Then go to the advanced system settings and then click on Settings in the Performance area. Click on the Advanced tab there and then there is a section called Virtual memory. Here you can click on Change and for your C drive you can click on Custom size and make the minimum and maximum values higher than what Windows has them set for. If you have other hard drives you can make a pagefile for those drives as well. If you are low on hard drive space then this may not help you.