Has Windows ever rudely informed you that your computer is low on Virtual Memory? If so, you have run out of swap file space. What Microsoft calls “Virtual Memory” is known as the swap file to the rest of the computing world. No matter what you call it, the swap file is a file or partition on the hard drive that can act as extended system memory if your physical amount of RAM gets full. A hard drive acting as RAM is both good and bad. On the one hand, the amount of programs your machine can load simultaneously is greatly increased. On the other hand, hard drives are much, much slower than RAM (by about 3 orders of magnitude, if you want to get really technical).
Generally, the size of your swap file should equal double the amount of RAM your system has. Windows manages your swap file for you by default, while Linux/Unix requires you to set one up during the initial installation. Even if you have a large amount of RAM, it is recommended that you not disable your swap file, since modern operating systems are designed to take advantage of it.