Today's computers are so powerful that users are not concerned or informed about virtual memory. Actually they don't need to! Because most of the modern computers comes with sufficient main memory or RAM. It may be 2 GB or more. And a PC with higher than 2 GB RAM hardly requires to use virtual memory.
A minor part of your hard disk is used as Virtual memory. Generally Windows OS automatically uses some space of your installation drive (In most cases Drive C). You can change the amount of virtual memory by yourself.
If you're running a program that requires a huge amount Random Access Memory, and if your computer lacks sufficient memory, Windows uses virtual memory to run that program.
In fact, virtual memory is used to boost the capacity of RAM and to accelerate the operations of your computer. It combines the RAM with temporary space on your hard disk. When RAM is low, virtual memory transfers data from RAM to a space named Paging File. Transferring data to paging file frees up the RAM to perform its task.
The more RAM you've on your PC, the faster you can operate it. So, if your PC is slowing down due to the lack of RAM, then you should increase the amount of virtual memory by compensating a part of your installation drive. This part or virtual memory will be used by Windows as RAM.
But increasing the virtual memory is not the ultimate solution. Because it is not as fast as RAM. And your PC can't read data from hard disk as quickly as it can read from RAM. So adding RAM is a better solution.