Today's modern operating systems all use a trick called virtual memory to feed your applications the memory that they need. Suppose that your PC has only 64MB of random access memory (RAM) installed, but you've just run Photoshop CS2 and demanded that it load a 30MB high-resolution digital image. If Windows XP were limited to using only your computer's physical RAM (the memory modules that you've installed on your PC's motherboard), you'd be up a creek because Windows XP requires a minimum of around 24MB of memory itself, and Photoshop CS2 takes a significant chunk of memory to run. And on top of all that, you're loading 30MB of data, too! With the size of today's documents and the amount of RAM needed by memory-hungry mega-applications, your 64MB PC literally can't do its job. And don't forget, you're probably running more than one application at once. What's a computer to do?
As you can see in Figure 1, Windows turns to your hard drive for help. It uses a portion of the empty space on your hard drive to temporarily hold the data that would otherwise be held in your computer's memory. In this case, our hardworking silicon warrior uses 64MB of hard drive space, so the total memory available within Windows (using both 64MB of physical memory and 64MB of virtual memory) is now 128MB, providing more elbow room to work with. Your programs actually don't know that they're using virtual memory - Windows takes care of everything behind the scenes, so Photoshop CS2 thinks that you have 128MB of physical memory.
Figure 1: Windows XP creates memory space from the free space on your hard drive.
You should always leave enough empty hard drive space for Windows to use as virtual memory. How much is enough? You might want to try leaving at least 1 or 2GB free on the C: drive at all times on a Windows XP machine. A PC that runs out of hard drive space is a terrible thing to see; applications will start to lock up, you might lose any changes that you've made to open files, and Windows will begin displaying pitiful error messages begging you to close some of your open application windows (or even restart).
Also, note that virtual memory is always slower than true physical memory. After all, that data has to be written to and read from your hard drive instead of super-fast memory modules. This is why you should add as much RAM to your PC as possible; the more memory that you add, the less likely that your operating system will need to resort to virtual memory.
PC techs call your computer's use of virtual memory drive thrashing because Windows must constantly write to, read from, and erase data from your hard drive. When you run out of physical memory, the hard drive activity light never seems to go out. And yes, all that activity will shorten the life of your hard drive.
