If your programs are working very slowly on your computer, the issue may be a virtual memory that is set too low. Virtual memory uses what is called a paging file to free up random access memory (RAM) on your computer. Your computer can operate better with more RAM available to complete tasks. Since virtual memory settings have such an impact on your computer's performance, you'll need to evaluate a few things before you reset the virtual memory.

Programs and Usage

If you are a multitasker who tends to have about a dozen programs open all at once, you'll need more virtual memory than someone who runs only two or three programs at a time. Take a look at the specifications for each individual program. Programs that are heavy on graphics (including video games) usually require quite a bit of virtual memory. Look on the box the program came in for how much RAM is recommended for each program, or use the Task Manager to view the amount of computer resources being pulled by the program while it is in use. Set the virtual memory to handle the programs you use most often, or set it based on the specific application you need to use at the moment.

Size Recommendations

Microsoft recommends that you use the amount of RAM you have as a basis for your virtual memory settings. They suggest putting one-and-a-half times the amount of RAM as the lowest setting and three times the amount of RAM as the maximum. More or less virtual memory may hinder your computer's performance. To find out how much RAM your computer has before setting the virtual memory, right-click on the "My Computer" icon on your desktop (you also can do this by right-clicking "My Computer" under the "Start" menu) and select "Properties." The "General" tab will show how much RAM the machine has. If this number seems inaccurate for whatever reason, you can always open up the computer tower and check the physical sticks of RAM in terms of number and size.

Restart

When you set virtual memory, you change the configuration settings. In Windows, configuration settings sometimes don't get applied until after you restart your computer, so close out of all your applications before setting the virtual memory so that you won't lose any work. However, Windows doesn't always prompt you to restart, which can make you think the settings were applied when they really weren't, which in turn can make you wonder if something else (e.g., bad sticks of RAM) aren't creating problems. Only start troubleshooting outside of the virtual memory settings if the problem persists after the restart.

Source:

Microsoft: How to Set Performance Settings in Windows XP