This behavior can occur for any of the following reasons:
Programs may be started automatically when you start your computer. Programs that run when you start your computer typically run all the time; this uses a portion of your computer's system resources that cannot be used for any other task.
You may be running a program that creates memory leaks. When you quit a program, the system resources that the program uses should be returned to the operating system. However, some programs do not return all of these resources, effectively "leaking" memory, and this can create a low system-resource state.
Your computer may have a small or minimal amount of random access memory (RAM), or a slower central processing unit (CPU). For example, although Windows XP can run with a minimum of 64 megabytes (MB) of RAM, this amount of RAM may not be sufficient to maintain a high speed while you run one or more programs. Also, if your computer has only the minimum CPU requirements for Windows XP (300MHz or higher), it may not be sufficient to maintain a high speed while you run one or more programs.
Verify that any programs that start automatically when you start your computer are truly needed and, if not, quit them. For example, an antivirus program is a program that you probably want to run all of the time, but you may have other less essential programs that you do not need to have running all the time.
To determine which programs are running, press CTRL+ALT+DELETE, click Task Manager, click the Applications tab, and note of all of the programs that are listed.
To quit a program, click the Applications tab, click the program that you want to quit, and then click End Task.
To permanently prevent a program from starting automatically, view the following article in the Microsoft Knowledge Base:
Determine if you are running programs that create memory leaks. Your computer may be leaking memory if your computer becomes slow after you run and quit one or more programs. If your computer runs well before you run a program, but then slows noticeably after you quit the program, the program may have a memory leak.
For more information about using performance monitor to identify memory leaks, click the following article number to view the article in the Microsoft Knowledge Base:
To resolve this issue, contact the manufacturer of the program to inquire about the availability of a fix for this issue. To work around this issue, restart your computer after you quit the program.
If you are using the minimal amount of RAM or a slower CPU, you can add more RAM to your computer or obtain and install a faster CPU. For information about how to do this, contact your computer manufacturer, or view the documentation that is included with your computer.