What is the difference between these two memories? I guess the virtual memory is only used by Windows, are there other operating systems that uses virtual memory?

Physical Memory is the from the hardware called RAM. While the virtual memory is exactly the word virtual which means its not real, because it uses the space in the harddisk to create some memory space.

The problem with virtual memory is that when you have allocated exact number of bytes and then your harddisk gets low on space, your system will have errors. Just leave the system allocate the virtual memory that it needs so you won't have any problem.

Virtual memory is a memory management technique, used by multitasking computer operating systems wherein non-contiguous memory is presented to software as contiguous memory. This contiguous memory is referred to as the virtual address space. It is used commonly and provides great benefit for users at a very low cost. The computer hardware that is the primary memory system of a computer also called as RAM is the physical memory.

RAM is the amount of physical memory installed on your computer. It acts like short-term memory, enabling programs to load and operate faster. When you start a program or an application, it uses a part of that memory. Most programs will require at least 32MB of RAM. If you look in your system tray (next to the time), there are usually four or more programs running (antivirus, firewall, and so on). Those background programs alone require 128MB. Then if you use any programs, such as a Web browser or a word processor, it requires more memory.

Windows makes up the difference for that amount of RAM requirement by allocating or reserving a portion of your hard drive to act as though it were RAM. That's what is referred to as virtual memory, or page file. The less RAM you have the more virtual memory is allocated. Windows typically manages that for you. As a standard rule of thumb, Windows creates a virtual memory of 1½ times your physical RAM. So if have 256MB of actual RAM, Windows pagefile (virtual memory) will equal 384MB.

RAM is the amount of physical memory installed on your computer. It acts like short-term memory, enabling programs to load and operate faster. When you start a program or an application, it uses a part of that memory. Most programs will require at least 32MB of RAM. If you look in your system tray (next to the time), there are usually four or more programs running (antivirus, firewall, and so on). Those background programs alone require 128MB. Then if you use any programs, such as a Web browser or a word processor, it requires more memory.

Windows makes up the difference for that amount of RAM requirement by allocating or reserving a portion of your hard drive to act as though it were RAM. That's what is referred to as virtual memory, or page file. The less RAM you have the more virtual memory is allocated. Windows typically manages that for you. As a standard rule of thumb, Windows creates a virtual memory of 1½ times your physical RAM. So if have 256MB of actual RAM, Windows pagefile (virtual memory) will equal 384MB.